Just to make sure it's clear here as well, the build GrahamO mentioned is only needed to play the Blue Planet mods. BP is designed to work with a special experimental build that uses cutting-edge features. The build I provided is a compatibility build for BP OS X players. For FS2 retail and all other FS2 mods, though, the standard OS X builds should be used, such as the official builds and the
nightly builds. Using a recent nightly build is recommended for now, because as Yarn said, 3.6.16 has a difficulty bug that's fixed in nightly builds.
EDIT: Irish29, you might also want to try disabling shaders in the launcher (check "Disable GLSL (shader) support" under "Troubleshooting" in the launcher's advanced settings tab) and seeing if that helps. Try generating a log as Yarn suggested with and without shaders enabled.
EDIT 2:
For the log, I do not have the folder ~/Library/FS2_Open/data **. Where else might the log while be?
The ** was just to indicate that there was a note at the bottom of that post.

The ~/Library folder is hidden in OS X Lion and Mountain Lion. See
here for how to temporarily unhide it.