Under the data folder, you have a Scenery folder, coming with the FG pack, containing itself Airport, Objects, Terrain folders (for San Francisco).
You could add (not replace) the content of your new downloaded folders inside the respective FG folders of the same name (they already contain w130n30 folders, observe them and their content). Your new folders must have the same structure, but contain your new Terrains and Objects files. Their content can be added to (not replace) the existing one.
For clarity, I have preferred creating a new folder (e.g. Scenery-Germany) under the FG data folder, and put my new content (Airports, Objects, Terrain folders) in it. I guess (not sure) that this new Scenery-Germany folder can even be anywhere else, even not in the FlightGear one (to preserve it and make it easier when you update or if you have to reinstall FG).
After that, you have to add the address (exact path) of this new folder in the first page of the Launcher (Add... FG_Scenery, in Windows, I don't know for Mac).
I hope this can help you at least to understand , despite my Windows explanations.
Last edited by dany93
on Mon Apr 18, 2011 4:07 pm, edited 1 time in total.