On the customer information page of checkout (first page of checkout) enter your coupon code in the discount code box and click apply. If you are returning to your cart make your way to the last page and the discount code box will be there. If the coupon does not work, please refer to the FAQ question above this one ^
From our home page, click 'contact' on the navigation bar and feel free to contact us about your problem. We will get back to you within 24 hours. We will then ask for details about your order, and sort this out for you.