If your a pCars member, when you look at your profile page at there web site, you'll see a steam key, enter that into your games/add steam key task bar within Steam and pCars will then begin downloading, in your steam account.

Supposedly better for updates/patches and user created content.
pCars is still in pre-alpha I believe, a race game by Slightly Mad Stidios that looks and sounds incredible, but still needs serious work on AI and presentation. BUT still a alpha build.