在Visual Studio中，Target IOS Device下拉框是禁用状态，无法选择。

Xamarin论坛中有不少关于这个问题的，如下面这个帖子：

http://forums.xamarin.com/discussion/15372/visual-studio-ios-debug-build-failure

解决办法就是在MAC端，卸载xamarin全部软件，然后重新安装：

If you are still running into this error, can you try fully uninstalling Xamarin Studio from the Mac Build Host and re-installing? To uninstall the software, please remove Xamarin Studio from /Applications/Xamarin Studio.app and then manually delete the the following -directories /Developer/MonoTouch

/Developer/MonoAndroid

/Library/Frameworks/Mono.framework

/Library/Frameworks/Xamarin.Android.framework

/Applications/Xamarin Studio.app

~/Library/Developer/Xamarin

To re-install, you can run the installer from your Xamarin Store Account

After re-installing, please let me know if you are unable to deploy to the iOS simulator. Thanks!

Xamarin Visual Studio无法debug