In Steam, add the game to your library by clicking the Games menu (top bar) and choosing Activate a Product on Steam
Paste in your Steam key to add the game
In the library, right click on Battle Chasers: Nightwar and choose Properties
Select the Betas tab
Paste the following code into the field and click CHECK CODE: k8RULa2aJnaY
Change the drop down above the code field from "NONE - OPT Out" To "Alpha"
The Alpha build should begin downloading!