A separate installation needs to be run to install the Xamarin.Mac package. Once that is done when you try to open a MonoMac project in Xamarin Studio you will get a button asking if you would like to migrate the code to Xamarin.Mac.
To get this download, sign in with your account on the Xamarin website, then in your account click on the Downloads link and you will see the OS X download for Xamarin.Mac.