In the latest Stable Xamarin Studio (6.1.2) there is a problem with add-ins - the add-in gallery is empty - the add-in endpoint for 6.1.2 is not returning any add-ins.
This means if you are planning on using my Xamarin Studio MvvmCross add-in then you won’t be able to find it.
Luckily there are a number of workarounds:
Use the Alpha channel - on 6.2 all the add-ins reappear.
Add the add-in repository from the previous version by opening the ‘Add-in manager’, going to the ‘Gallery’ tab, selecting ‘Manage Repositories’ from the ‘Repository’ drop-down, tapping ‘Add’ and entering this url - http://addins.monodevelop.com/Stable/Mac/6.1.2/main.mrep
Install the add-in manually by downloading from here and installing it using the ‘Install from file…’ option in the ‘Add-in manager’.
This issue has been raised with Xamarin, so hopefully should be fixed soon.