if you have been provided with a promo code for, you will have a couple of options to enter it while checking out. before attempting to use a promo code, be sure to read the terms and conditions for that code, including expirations dates, combinability rules, and eligible products.
on the shopping bag page

click the "have a promo code?" button and enter the requested information.

on the payment page
expand the "have a promo code?" field by clicking on the "+" sign and enter the requested information.
if your discount qualifies, it will be displayed in your shopping bag and in your order summary throughout the checkout process.