You should better use open-source MonoDevelop instead:
Well, the latest version. Yes, I used it on Mac OS X (on Snow Leopard though; but it will work on your system).
For Cocoa development, you need to use MonObjC:
You would need to use XCode if you develop native applications. As you use Mono, you are developing code for CLI which is implemented in Mono, and here you can use MonoDevelop.