Of course, you need the Razer Hydra motion controller device itself to be able to play this content. And you can get that through Steam, as well. No, really. You can order a controller through Steam. Who knew? The controller comes with the DLC and a free copy of Portal 2. Which you'll likely be giving away, if you're the Portal-crazed person we mentioned at the top of the post.
[Thanks, Jason G.!]