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.