Manually install Xamarin Studio addins

Just copy the .dll file to (note the Xamarin Studio version, it might be different today):

on Windows:
%LocalAppData%\XamarinStudio-5.0\LocalInstall\Addins

on Mac:
/Users/[YourUser]/Library/Application Support/XamarinStudio-5.0/LocalInstall/Addins

Source:
http://forums.xamarin.com/discussion/comment/6356/#Comment_6356