Mono for Android does not currently bundle a UI designer to create the UI XML files, but you can use DroidDraw (http://www.droiddraw.org).

We are in the process of creating a UI Designer to compliment Mono for Android.  This effort is still in the beta stage, but we welcome our users to participate in the open beta if they'd like to get an early glimpse of the designer and provide feedback.  To sign up for the beta program, please follow the link below:

http://blog.xamarin.com/2012/03/22/mono-for-android-designer-beta-program/