I was pretty sure at one point I came across a site with a gaim port to darwin.....maybe I imagined it. But if I didnt and someone knows where it is please share the wealth.
The only way I know to install it is through Fink. You'll have to copy the gaim.* files from sw/fink/dists/unstable/main/finkinfo/net to sw/fink/dists/stable/main/finkinfo/net in order to get it to run. The main reason I'd suggest trying it with Fink (other than not knowing any other way to get it) is because you'll be able to get plenty of other programs that way, too.