Finish the sorcerer's ordeal then complete carnatux's sympatha's and red hood's respective chapter's to unlock multiplayer. In the vanilla version, the Ordeal was all you had to do, now they ask for you to complete a bit of the expansion.
Edit: Don't quote me on this but i believe they do that to introduce to you the new mechanics and systems the game has to offer. So enjoy the story its a great game, Good Gaming.