----- Download and install Steam http://store.steampowered.com/ (if not already installed)
----- Register a new account on Steam or enter the existing one
----- Go to "My Games" and select "Activate via Steam ...", read and accept the Subscriber Agreement of Steam and enter Darkness 2 cd key
----- After activating Darkness 2 cd key, Darkness game appears in the list of games and you can download Darkness 2 from Steam
Then, enjoy the game!