Well, go through the steps of what you tried to do, and we'll try to help.
I don't own a Mac, but two people searching through google won't hurt. :p
And here is the home page to NetRadiant:
http://ingar.satgnu.net/gtkradiant/index.htmlI use GTKRadiant, for the time being I suppose. It suits me fine, but NetRadiant seems more updated. (Don't know for sure, although)
And I'm sure Mapping tutorials are all over Youtube, which I heard the "hip" people like these days.