If you see this message in VS2012 with Xamarin.iOS working with MvvmCross
Unable to add a reference to project 'MyApp.Core'. The current project does not support references to Portable Library projects.
then this may be because you have not yet got PCL support added for MonoTouch (Xamarin.iOS)
To add this, create a new file
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETPortable\v4.0\Profile\Profile104\SupportedFrameworks