Just the other day I downloaded mono to investigate it but....
When creating a new c# solution I choose a gtk# 2.0 Project which in turns displays a window "project features" where gtk# support version 2.12.9 is the target version.
When I open the MainWindow.cs file the source code is displayed but there is no "Design" tab and no widgit palette nor widgit properties panes.
I have checked via synaptic that all the gtk-sharp related modules are installed.
I have checked via monodevelop menu tools > add-in manager that the IDE Extensions - GTK# Visual Designer is installed.
As far as I can tell everything is there but monodevelop is just not giving me the GUI design facility.