To get PCLs working for the Xamarin Twins (MonoTouch and Mono for Android) in Visual Studio - the only current changes I make are these two files which I add to:
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETPortable\v4.0\Profile\Profile104\SupportedFrameworks
If you have previous files added - especially for VSMonoTouch - then please remove these .xml files as they may cause issues.
Following Evolve, we *hope* that these additional files won't be needed soon - hopefully in the next 4 to 8 weeks. Please bare with us - the best cakes take time to bake :)