Promotional codes, including coupons, must be entered on the payment selection page as part of the checkout process. You will see a field near the bottom of the page for "Discount Code". Enter the code you have in that field. A green check mark will appear if the coupon code is valid and can be used. A red "X" will appear if the coupon code is invalid or cannot be used.
Please note that you must have a product that is discount eligible for the coupon to be applied and the green check mark to show. If there are no eligible products in your cart the discount code field will still show a red "X" even though the code itself if valid