You can add promo codes in two places, both on the Cart and on the Checkout page.
On the Cart page, enter your promo code in the "Coupon Code" field, and click the "Apply Coupon" button.
On the Checkout page, click the "Click here to enter your code" text at the top of the page, enter your promo code in the "Coupon Code" field, and click the "Apply Coupon" button.