Content Pack

[King Kickass]

I don't have a Mac but I'm guessing you
Go to ~/Library/Application Support/Steam/SteamApps/[your username]/garrysmod/garrysmod/
Taken from the internet

Ok, but what about the extracting?

Well one you have downloaded this

http://fearlessrp.net/content

I belive you just either unzip it with os x and place the extracted file into the  add ons folder.  Or you just place the file as it is into the add ons folder