Atsuke:
Welcome to the FlightGear discussions.
When you download scenery from flightgear.org, it is not initially readable by the program.  What I do is, after installing it to my desktop, I use the fgadmin program located in C:\Program Files\FlightGear\bin\win32.  (I have a shortcut of the program  on my desktop.)  I do not personally use TerraSync, but you can download it froom cvs.flightgear.org/cgi-bin/viewvc/viewvc.cgi/source/utils/TerraSync/.  After you download the tarball, you will need the 7-zip program to decompress it.  That program can be found at www.7-zip.org/.  I hope this helps.
Josh