1) You must download and install Steam http://steampowered.com/download/SteamInstall.msi (if not already installed).
2) Create a new account on Steam or go to an existing one.
3) Select "Activate via Steam ..." in the "My Games" section, and enter the key received after payment.
4) After activation, the game will appear in your list of games and you can install it.
Please leave feedback after successful activation of the game.