High on Knife DLC not starting

[John]

I bought the High on Knife DLC with the game and I've completed the main game. When I'm in the main menu, High on Knife is green but when I click on it nothing happens. I can get in to the normal game perfectly fine, but I've tried a few times to get into the DLC but it just won't start. Anyone got any ideas what I can do to play it?