Under options, enter code. EDIT: Just figured it out. Needed to select the gun and add the code there. Sorry!
Just wanted to pop in real quick and say that the contest is off to a great start and when we get to the first 100 (we are at 40) we will be picking the first winner out of a hat and have a video showing us doing it! Also, we're going to try and change the font for the gun code in the next update so that the 0 and the O aren't so close.