At the moment, the current version of Xamarin.Mac installed by the Xamarin Universal Installer is not an up to date version. This version also contains a bug that may cause it to freeze or throw error codes during activation. The Xamarin.Mac team is currently aware of this and working on fixing it.
While the Mac team is addressing the issue in the Universal Installer, a newer version of Xamarin.Mac that solves the activation error can be manually installed from the following address:
Issues with Installation and Activation of Xamarin.Mac
Last Updated: Nov 01, 2016 05:36PM EDT