To use your affiliate credit, you will first need to add a product to your cart and proceed to checkout. On the page where you choose your payment option, there will be an option just above the credit card form that says "Check to use account credit ($XX.xx)" - see below:
Simply check the box there, and your account credit will be used toward that purchase, you will see the order total adjusted on the next checkout page. If your account credit completely covers the purchase amount, you don't even have to enter any credit card information. Otherwise, your credit card will be used to pay the difference.