I was running Eclipse Neon.2 and the Android SDK Build-tools + platform-tools version 26 on Mac OS 10.12.4 and none of the above answers (including the accepted answer) worked for me.

What did work was to

1. Quit Eclipse

2. Remove the folder <android-sdk>/build-tools/26.0.0 and to install the (older) version 25.0.3 of the build tools through the Android SDK manager.

3. Start Eclipse again

Android ADT error, dx.jar was not loaded from the SDK folder