Without the help of an expert, I would suggest you throw away the FG version you already have, then download a fresh one from the Mac OSX page: http://macflightgear.sourceforge.net/home/downloads/
I suggest this because as far as I know, the Mac downloads have always worked straight from the box (figuratively speaking).
It is always possible that you didn't get a completely clean download. It can take a while, and it is tempting to continue using your computer before it is finished (always possible to mess things up), but I have always done any new version download overnight, so I can leave my Mac to get on with it.
That way the installation is so straightforward that PC users get jealous
Personally, I don't download anything else until I have made sure it works. I also don't download any scenery, but use the tick box on the front page of the launcher which allows the scenery to be loaded on the fly. This is only a chore when you want to launch from an airport where you have not yet flown close enough to pick up the scenery. In that case when you launch you will find you are sitting on top of water. You just have to wait for a minute, then cease FG and start again - this time the scenery should be there (maybe you have to do this twice sometimes).
If you have already reached the stage where you are not too fussed if it won't work, a fresh download is worth a try. As you can see, I have not given up on you yet.