Bridge Constructor Portal DLC

How do I buy it? I'm playing the game from the disc but I'd really like to get the last 170. But the DLC isn't listed in the Xbox store and it gives me an error when I click the "Portal Proficiency DLC" button in the disc menu. Can I only get it by purchasing the full game from the Xbox store? Someone played it because it says 0.07% completed the first achievement. I'm totally confused.