If you have a promotional code, you can apply that towards your order in your basket or on the billing address page.
From your basket, you will find a field to enter your promotional code beneath your total. On the billing address page, there is a link underneath the payment method fields that says, "Have a coupon or gift card?" Clicking on that will open the hidden coupon code and gift card fields. Enter your promotional code exactly as it appears, including any capital letters or dashes, and then press "Apply Coupon" to redeem.