'...are you missing a using directive or an assembly reference?'
I tried the solution in this question Gtk# in monodevelop not working anymore after upgrading to ubuntu 11.10 unity but appears to not work here.
I am running on Mtn Lion with lastest MonoFramework 2.10.10 and MonoDevelop 3.1.1
Any suggestions to get build going? Thanks.
The error message says exactly what you need to do.
using Gtk;
to your code.