Actually, it's coming out in like 3 days, I think! (I got an early release key and played with the multiplayer for a couple hours: multiplayer is cool! I've never done any online game before, and even though we were just running around shooting each other, it was fun! Damn you, Slaur3n!)
Anyway, once you get your key, open your Steam client (do you have it installed? If not, do it! I'm really not sure if you can do this from a browser), click on "Games" at the top, then click on "Activate a Product on Steam...". Click next on the first window, Agree to agree, thenon the next window I think it asks for the key.
3days, 15 hours, just confirmed! February 27th!
MAME Cab PC: i7 4770@3.4Ghz (passmark 9945), 12GB RAM, Win 8.1/64, GeForce GTX645 (passmark 1898); Shiny new laptop: i7 4800MQ@2.7Ghz (passmark 8586), 16GB RAM, Win 8.1/64, GeForce GTX870M (passmark 3598); Old laptop: i5@2.3Ghz, 8GB RAM, Win 7/64, Intel 3000 graphics