I don't use a mac, but I assume that it's pretty similar. Glest should be installed as glest_* (such as glest_3.2.0). This is usually installed in a directory for programs (ie: program files). By navigating to this directory, you'll see a bunch of folders. In the techs folder, you can put new tech trees. New factions can be implimented into Magitech by opening the folders techs>magitech>factions.
Tilesets are found in the tileset directory in the glest directory, and scenarios in the scenario folder. Maps in the maps folder. That's pretty much everything, though feel free to ask if you're having more trouble.