If you're diving into building cross platform apps using Xamarin and you need to use Active Directory Authentication Library, you will need to create bindings so you can call the native iOS and Android libraries from C#. Xamarin describes the process of creating bindings libraries in details here (Objective -C) and here (Java). But if that feels like too much work, the AD folks have published sample codes that will get you going:

Sample binding project for Xamarin iOS

Sample bindings for Xamarin Android

I will blog more about getting these things to work later on.