Never had a problem with grey scale height fields on a Mac. Make sure when you save it out it's just a plain PNG or JPG - Torque on the Mac doesn't like things like "Progressive Download"s and the like, and will crash or hang Torque on loading the texture.
Send me the original file and I'll see if I can find out what's going on.
Torque 3D Owner James
Send me the original file and I'll see if I can find out what's going on.