A guildy and friend recently switched to the Mac, and being a programmer, he wrote himself a handy little app to automatically download and update ACE mods. It works along similar principles as the PC WoWAceUpdater, although I personally find it much more reliable.
I asked if he'd mind me sharing it, since I know there are other folks out there who play on a Mac, and he gave me his blessing, so here it is. This page has the download link and some documentation I threw together for it. It is, of course, free, and provided as-is.