Many of you do not know how to do this. So i will explain! First: select 'games' in this area of the steam menu: [attachment=2494:8bitmmo guide 1.PNG]. Then, press 'activate a product on steam' as shown here: [attachment=2497:8bitmmo guide 2.png]. Agree to the Steam Subscriber Agreement and enter the code in this screen: [attachment=2496:8bitmmo guide 3.PNG].
If done successfully, 8bitmmo will now be downloading! (TIP: Click on the images to see them in a bigger view because its hard to read).
EDIT: 8bitmmo is free to download now. But i am leaving this up in case someone wants to activate another game using this way).