After ASP.NET 3.5 has been installed you need to modify the web.config file of your MOSS web site with a few Ajax specific entries. Typically, the web.config file is located in c:\inetpub\wwwroot\wss\virtualdirectories\80

1. Add the following <sectionGroup> element in the <configSections> tag:

<sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup,
    System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
    <sectionGroup name="scripting" type="System.Web.Configuration.ScriptingSectionGroup,
        System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
        <section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection,
            System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" 
            requirePermission="false" allowDefinition="MachineToApplication"/>
        <sectionGroup name="webServices" type="System.Web.Configuration.ScriptingWebServicesSectionGroup,
            System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
            <section name="jsonSerialization" type="System.Web.Configuration.ScriptingJsonSerializationSection,
                System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" 
                requirePermission="false" allowDefinition="Everywhere" />
            <section name="profileService" type="System.Web.Configuration.ScriptingProfileServiceSection,
                System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" 
                requirePermission="false" allowDefinition="MachineToApplication" />
            <section name="authenticationService" type="System.Web.Configuration.ScriptingAuthenticationServiceSection,
                System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" 
                requirePermission="false" allowDefinition="MachineToApplication" />
            <section name="roleService" type="System.Web.Configuration.ScriptingRoleServiceSection,
                System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" 
                requirePermission="false" allowDefinition="MachineToApplication" />
        </sectionGroup>
    </sectionGroup>
</sectionGroup>

2. Add the following <controls> section as a child of the <system.web>/<pages> tag:

<controls>
    <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral,
        PublicKeyToken=31BF3856AD364E35"/>
    <add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral,
        PublicKeyToken=31BF3856AD364E35"/>
</controls>

3. Add the following tag to the <assemblies> tag, within the <compilation> element:

<add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Data.DataSetExtensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>

4. Register the following HTTP handlers at the end of the <httpHandlers> section:

<add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory,
   System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory,
   System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler,
   System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" validate="false"/>

5. Add the following HTTP module registration to the <httpModules> section beneath any existing modules:

<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, 
    Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

6. Add a SafeControl entry for the System.Web.UI namespace from the System.Web.Extensions assembly within the <SharePoint>/<SafeControls> section:

<SafeControl Assembly="System.Web.Silverlight,
           Version=2.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
           Namespace="System.Web.UI.SilverlightControls" TypeName="*" Safe="True" />
<SafeControl Assembly="System.Web.Extensions,
           Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
           Namespace="System.Web.UI" TypeName="*" Safe="True" />

7. Since the RadEditor dll files, which are in the wsp package, are complied for .NET 2.0 framework, you need to add the following code to the <runtime><assemblyBinding> section.

<dependentAssembly>
    <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>
    <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
</dependentAssembly>
<dependentAssembly>
    <assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35"/>
    <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
</dependentAssembly>

8. Finally, add the following configuration tags at the bottom of web.config, just before the end of the <configuration> tag:

<system.web.extensions>
     <scripting>
     <webServices>
     </webServices>
     </scripting>
 </system.web.extensions>
 <system.webServer>
   <validation validateIntegratedModeConfiguration="false"/>
   <modules>
     <remove name="ScriptModule" />
     <add name="ScriptModule" preCondition="managedHandler"
        type="System.Web.Handlers.ScriptModule,
        System.Web.Extensions, Version=3.5.0.0, Culture=neutral,
        PublicKeyToken=31BF3856AD364E35"/>
   </modules>
   <handlers>
     <remove name="WebServiceHandlerFactory-Integrated"/>
     <remove name="ScriptHandlerFactory" />
     <remove name="ScriptHandlerFactoryAppServices" />
     <remove name="ScriptResource" />
     <add name="ScriptHandlerFactory" verb="*" path="*.asmx"
        preCondition="integratedMode"
        type="System.Web.Script.Services.ScriptHandlerFactory,
        System.Web.Extensions, Version=3.5.0.0, Culture=neutral,
        PublicKeyToken=31BF3856AD364E35"/>
     <add name="ScriptHandlerFactoryAppServices" verb="*"
        path="*_AppService.axd"
        preCondition="integratedMode"
        type="System.Web.Script.Services.ScriptHandlerFactory,
        System.Web.Extensions, Version=3.5.0.0, Culture=neutral,
        PublicKeyToken=31BF3856AD364E35"/>
     <add name="ScriptResource" preCondition="integratedMode"
        verb="GET,HEAD" path="ScriptResource.axd"
        type="System.Web.Handlers.ScriptResourceHandler,
        System.Web.Extensions, Version=3.5.0.0, Culture=neutral,
        PublicKeyToken=31BF3856AD364E35" />
   </handlers>
</system.webServer>