You will be able to use MonoDevelop for the Mac OSX. Go to Mono.com here and you will find the information on how to install it on Mac OSX. The download page for Mac OSX is here, click on the Mac OSX icon and you should be good to go after downloading the appropriate packages.
There is a caveat emptor which I'll quote from the page - Gtk# and System.Windows.Forms applications require X11. Installing on a machine without X11 installed will result in errors during install, and these components will not function correctly.
I would guess it has to do with the Mac OS X's GUI conflicting with X...I'd imagine..not 100% sure.
For MonoDevelop, click here, click on the Mac OSX Icon..this will sit on top of the Mono framework...you will need to dig around a bit to see if this will co-exist with your existing setup... not sure here...
The good news on the Linux side, is it works, if you are building mono from scratch, using the Mono-2.6.1. sources here, build in this order...
- mono-2.6.1.tar.bz2
- libgdiplus-2.6.1.tar.bz2
- mono-tools-2.6.1.tar.bz2
- mono-debugger-2.6.1.tar.bz2
- mono-addins-2.6.1.tar.bz2
The rest of them are optional.
Edit: I have added a few more bits of information. There is a cross-platform alternative to designing GUI's with Mono, it is Gtk# and Glade#. The Gtk# toolkit is a wrapper around the GTK tools that is used by Gnome. Glade# is a UI designer toolkit that uses Gtk#. This answer would not be complete without mentioning a couple of links to some tutorials on Gtk#.
All of the above links reside at CodeProject.
Hope this helps,
Best regards,
Tom.