Ryan,
I'm not sure exactly what you mean by ``editing'', but if you mean ``adding'' and ``removing'' entries, I found ``Editing GNOME2 Menus in Red Hat Linux 8.0 and 9'', at
http://www.bluethingy.com/linux/rh8menu.html
to be pretty helpful in understanding how this stuff works. It seems like the information in that article applies to CentOS 3 (and, presumably, also to CentOS 4). (Yes, I happened to be tinkering with the menu stuff when I saw your message.)
Claire