Download it from steam: If your key is legit it will not cost you an additional fee.
That's just how steam works (It's wrong I know; but it is what it is)
You can only play Multi-player through steam; every one who buys a retail hard disc, is just paying the game shop dealer, to down load steam. (that's why i don't bother with hard disc for steam games any more) the one good point is you can never "loose" the disc...lol
If you don't have a steam account? and have the retail DVD game you will get a prompt as you install the game to create an account, if you only have an activation code you will need to down load steam and create an account, go to the menu and select activate game.
