If you are not able to build using Visual Studio for Mac because Android SDK, NDK and JDK seems to be missing, you will need to configure your preferences settings.

Configure your "preferences" setting in Visual Studio for Mac (Or Xamarin Studio)

Navigate to 'Preferences' and select the 'Locations' tab before copying in the location addresses below:

If the green "Found" checkmark is still showing red, please contact our support staff to receive updates to your server. 

Also make sure to select "Microsoft" as the source of Repository using the gear setting icon (As shown in the screenshot above). Otherwise the JDK setting will not stay after saving.