Bringing up a new PC with Xamarin and VS 2015 Pro I received the following error:
An unexpected error occurred trying to initialize Android Designer.
If you are already using Android Studio in your computer then this might solve your problem. Check whether SDK is already available at some other location. That worked for me. To check SDK path- Open Android Studio-> Click on SDK Manager Icon. Check Android SDK Location. Copy that path.
In Visual Studio-- Open Tools->Options->Xamarin->Android Settings menu and replace the new SDK path. It will automatically ask to download the updates. Install all packages and you are done!