In order to use your coupon code, just place the items into your cart and proceed to the checkout page.
On the checkout page (where you choose your payment method, directly below the "Payment Method section), will be a section for "Coupons Gift Certificates". Where it says "Enter Redeem Code", you should put in the coupon code that you were given in that box and then hit "Redeem" -- this will give you the discount amount off of your purchase. You will see the order total adjusted on the following screen at checkout.
If your purchase was for $0 then you will be directed straight to the download page. If your coupon does not cover the full cost of your order, then you will be asked to enter your credit card or PayPal information to account for the balance. Once done you will be taken to the download page.