In order to use your coupon, just decide what products you want 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.
If your purchase was less than the coupon amount then you will be directed straight to the download page. If your purchase exceeds the amount of available credit then you will be asked to enter your credit card or PayPal information to account for the balance.