GMod Crashing on startup (Mac)

When ever I start up GMod it crashes. I have a MacBook running OSX 10.6.4 with a 2.13 Core 2 Duo, 2 gigs of ram, and a NVIDIA GeForce 9400M.
I have Half-Life 2 downloaded and installed.
The steam “Launching game” window comes up, then closes. Then HL2_OSX opens and stops responding.
Help please!