and/or XamMac.dll in the same location, or,
(The Xamarin target may change next month when "Xamarin.Mac.dll" is released.) These I copied to MonoMac and XamMac folders in C:\Program Files (x86)\Reference Assemblies in Windows.
Then, in regedit, create keys
In each, assign the (Default) REG_SZ with the corresponding location in Reference Assemblies. For example:
Now when you Add Reference, you should see both MonoMac and XamMac.
Note that there is no need to involve the GAC.