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, click on the link 'Have a code?' just above the "Become a Member".
Once you enter the code, do not forget to click 'Apply'! Then you can click 'Become a member' to finish signing up.