Like you I am a Mac user with 10.5.8. Unfortunately, I only have 1GB of RAM, so I have to optimise as much as possible.
To find out which graphics card you have, Click on the Apple icon (top left of your screen) and select 'About this Mac. Then click on 'More Info'. Click on the triangle beside 'Hardware', then select 'Graphics/Displays'. There are your details. Most Macs have a built-in Graphics card, so you don't have the option to upgrade it.
It sounds like the downloaded scenery is not being put in the correct place within your FG files. Either that, or the path from your launcher to it is wrong. I will not try to tell you how to correct that because I could end up disappearing up my own exhaust pipe. However, there is an alternative way to get your scenery.
I no longer download the scenery to install. I have the 'Download scenery on the fly' box checked (front page of the Launcher).
Your scenery does not appear at your chosen airport the first time you launch, but it should be there the 2nd or 3rd try. With the 2.4.0 release, I found I had to launch at KSFO (the default airport) with the Cessna 172 (default aircraft) before Terrasync worked. That may have been fixed, but is worth bearing in mind if it doesn't seem to be working straight away.
That way your scenery is being updated with the latest versions each time you fly. If I think it is worth it, I bring forward my Scenery-Terrasync folder from the previous version of FG.
I hope you can make that work for you.