This is just a little observation and I hope that it might help someone.
I installed Xamarin Studio and Xamarin.Android to try it out. I usually do my Android development using Xamarin and Visual Studio, but today I figured I'd give it a try on Mac. Just to see how it works.
After installing and creating my first Android app I discovered that most of the framework assemblies couldn't be referenced.
The solution was simple however.
1. Go to Tools -> Open Android SDK Manager
2. Install the SDK Platform for any of the newer versions of Android that you'd like to target
3. Restart Xamarin studio
You might also want to check out the Target Framework setting under the project options.