I installed the mono CSharpPlugin for Xcode but I don't see an entry in the New Project wizard that looks like "CSharp Project" or something similar. My guess is that the plugin hasn't been developed lately and it doesn't work with the newer versions of Xcode, because I saw that the last update to the plugin code was back in 2007.
Is there another way to develop C# applications on a Mac, and benefit from testing, breakpoints, etc? I need this to work on a project for school.
Your best bet would be to install MonoDevelop (aka Xamarin Studio) which can be used to write Mono-based applications on Mac OS X. It comes with project templates for Mac apps that you can use to write open source Mac apps in C# (via the MonoMac binding).