I did not have any luck installing the extension to Visual Studio for Mac.
Someone from the Visual Studio community told me:
Visual Studio for Mac currently uses .mpack files which are .zip files that contain the extensions. This is a custom format that is different to Visual Studio on Windows.
Do you have a download for the .NET SDK for Mac?
On Mac, you will need to reference the NuGet package directly. See this help topic for more info. That documentation is for Visual Studio on Windows, but the steps on Mac are analgous. You can see Xamarin's documentation here for Mac-specific instructions. That document is technically for Xamarin Studio, but the UI is not substantially different in Visual Studio for Mac.