In order to use your coupon, just decide what products you want and proceed to the checkout page. Close to the bottom of the checkout page will be a section for "Credits Available". 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.
If you expected your coupon to make your purchase complimentary but you still get an error asking you to enter your credit card information, simply select PayPal as your payment method and then click Continue. The order confirmation screen will then verify for you that your order is complimentary and there will be no charge.