Eclipse. Start by looking here:
http://www.chiefdelphi.com/media/papers/1729
If you search around these forums, you should find all of the information you need to get it running.
If you're trying to use your mac to do all of your programming, you'll have to figure out how to get the C18 compiler to run on OSX. People have done it on Linux, but I don't know about mac.