Your referral/promo code is entered in the last step of the signup process, on the same page where you'll enter your billing information. To redeem your code, complete the signup process by taking the style survey and entering your shipping information. Once you arrive at the billing page, you'll see a place to enter your code just above the "Activate Account" button.

Once you enter the code, click the "Activate Account" button, and your code will be automatically applied!