/Applications/Xamarin Studio.app/Contents/MacOS/lib/monodevelop/AddIns
and/or XamMac.dll in the same location, or,
/Library/Frameworks/Xamarin.Mac.framework/Versions/Current/usr/lib/mono
(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
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727\AssemblyFoldersEx\MonoMac
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727\AssemblyFoldersEx\XamMac
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.
