If you have bought the game digitally you should receive a code directly from your retailer. If you have not, you need to contact them directly.
Once you have your activation code, do as follows to activate and start downloading your game:
- Download the Steam client: http://store.steampowered.com/about/
- Install Steam on your computer
- Once installed, open up Steam
- Go to +ADD A GAME at the bottom-left of the window
- Then select Activate a product on Steam
- Enter your pre-release Activation Code
- Steam should now download your game.
If you have bought a boxed version of the game, the code can be found on the back of the manual. If you put the disc in, you will be prompted to install the game using the code on the back of the manual. Use the instructions above to do so.