Well, I just used 'xmess' available from http://x.mame.net/
Now, the build process is a REAL pain. You have to edit a rather large makefile by hand, and must have the development tools from Apple (free, but a very large download)
It's updated to 0.92, however, so if you're used to building your own software under Unix, it's a good alternative.
Oh, you also need X11 for OS X. Comes with 10.3, or a free download from Apple.
I think one drawback to emulators and homebrewers is that the people creating them assume that the people who want to use the emulators have the same skillsets as they do.
Often, Mac users are Mac users because they don't want to know about "build pocesses", "develpment tools", "X11" and special tools.
They just want to click the button that says "install" and play.
Huh......No!
But then again maybe the reason there is a great 5200 for both PC's and Macs and there isn't a good 7800 emulator for either is because the 5200 is such a better and more desirable system to play. Sorry DracIsBack. I couldn't resist poking fun at you. Hey, hey, hey.
Allan