The easiest way is to NOT install the scenery at all unless you are on a dial-up connection.
So, provided you are using a broadband connection, all you need to do is tick the 'Load scenery on the fly' box, which is on the front page of the Mac Launcher.
The first time you try to launch from anywhere other than KSFO (the default airport) you will find yourself sitting on water with no scenery at all. Wait for a few seconds, say 30, then quit and re-launch FlightGear at the same airport. FG should then pick up the scenery for you as you fly towards it.
That way, you don't have to have the whole world's scenery using up space on your computer. FG will store where you have been in a folder called Scenery-Terrasync. I think you will find that you will fly in certain places because of things like scenery, or certain airports because you like them, etc. Having the whole world saved on your hard disc is a bit wasteful on your resources.
I hope that helps.
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