How do I develop a GUI for my MonoMac applications? I can't find any option in MonoDevelop.
You can only create a MonoMac application on Mac OS X within MonoDevelop, and then use Interface Builder (or Xcode 4+) to modify the GUI. More information can be found on the developers' blogs, such as
http://mjhutchinson.com/journal/2010/06/09/monomac_in_monodevelop
This is quite similar to MonoTouch development.