Stable release of Xamarin Studio 5.5.4.
That cannot be related to the version of Xamarin Studio. This is even unlikely to be related to the Xamarin.iOS version you're using.
Such MT2002 errors happens when your project is using a binary assembly (*.dll) that has been compiled against the desktop .NET framework (or Mono). It can contains references to methods, types or assemblies that are not available for the platform you're targeting.
Your assemblies must be compiled against either:
the mobile profile (what's shipping with Xamarin.iOS, Xamarin.Android and soon/alpha Xamarin.Mac); or
a PCL subset that is compatible with XI/XA/XM.
If you have something else then it needs to be re-compiled against the above.
