I understand how to download the adddon packs but on os x lion there is no application support folder so i dont know how to apply the map pack to gmod
I had the same problem for a while. What I did was open finder, then under places, select your username, then you should see a folder titled library. Open that one, and application support will be in that.
Open a finder window “you can also just click a blank part of the desktop to select finder”, then at the top go to the menubar option called “Go”, hold down the option key and a folder called “Library” should appear as a option. Click it and then follow this path
Library > Application Support > Steam > SteamApps > yoursteamname > garrysmod > garrysmod > addons