It's under the "add-ons" menu in game. I would recommend playing the top-ranked campaigns. Some amazing stuff in there. I adore Legend of the Invincibles. Fun story, tons of new gameplay mechanics.

There was (still is?) a Middle East themed campaign with just absolutely gorgeous artwork.

Last time I checked there was an option on the main menu to download user-made campaigns.

There's a "addons" browser in the game.