If you installed it properly it should work, so I suspect something is not quite correct.
Have you tried using Terrasync ?
All you need to do is click on the box next to 'Download scenery on the fly' on the front page of the Mac OS X launcher. Then start a flight.
The first launch may give you the same picture (sitting on water with no scenery), but if you wait for a couple of minutes, then quit and relaunch FG, you should have the scenery downloaded into your Terrasync - Scenery folder. You may have to do this twice, but it usually works on the first effort.
Let us know how it goes.