You can download Terrain from the FlightGear site : http://www.flightgear.org/Downloads/scenery-0.9.10.html
TODO : Install Help - fgadmin?
Objects and models can be found at the FlightGear Scenery Database
TODO : Where do I put these?
Scenery for FlightGear is determined by looking at the environment variable $FG_SCENERY. Should the paths in $FG_SCENERY not be valid, FlightGear falls back to $FG_ROOT/scenery.
Mixing scenery types is almost universally a bad idea, and is unsupported. In this connection you should note that the scenery packaged with the base package/windows installer may not be the latest scenery.
For this reason it's probably better to extract scenery files you've downloaded into a new folder, which we'll call, for the sake of this exercise, $FOOBAR/Scenery In this directory, create two subdirectories . /Objects and /Terrain. You should untar individual files into the /Terrain folder.
Then you should set up $FG_SCENERY to use the new path: On POSIX Clients set FG_SCENERY="$FOOBAR/Scenery" + FG_SCENERY On Windows (fgrun) Go back to the first page of fgrun, and add the $FOOBAR/Scenery folder to the list.