If you want free your only real choice is OpenMCL
it is not bad but it is still a little rough around the edges. It not a GUI app but all Lisp programmers know that Emacs+ILisp is the only GUI you will ever need.
If you are willing to part with a bit of coin Franz.com has a version of allegro Common Lisp for OS X. Finally the classic lisp for the Mac is MCL from digitool.com but it only runs in classic althought they promise an OS X version real soon now.
I am trying to install SWI-Prolog using the .mpkg from their site. I have xfree86 running. When I try to run it though, I get the following errors:
./swipl undefined reference to _BC expected to be defined in /usr/lib/libSystem.B.dylib
./swipl undefined reference to _PC expected to be defined in /usr/lib/libSystem.B.dylib
./swipl undefined reference to _UP expected to be defined in /usr/lib/libSystem.B.dylib
Trace/BPT trap