How do I get add-ons in Garry's Mod?

From what I gather, I'm supposed to download the file and then extract it to my steam/steamapps/username/garrysmod/garry… folder, but that doesn't seem to be working. When I go in-game, I don't see anything new under my spawnlists. Am I doing something wrong? How would I get this one, for example, into my game? http://www.garrysmod.org/downloads/?a=view&id=99852