I did this just a while ago... it's not easy to find.
On the cart screen, you have to click on proceed to checkout, do not go directly to Checkout with PayPal or Amazon Pay. You will need a Penn State account for this. (I'm not 100% sure you can't get there through the PayPal or Amazon routes but I couldn't find anything. If you want to use one of these payment methods, you will have a chance later to select it.)
Once you are signed it to your Penn State account, proceed through the screens until you get to the Shipping and Payment Options screen. At the bottom of the shipping section (just above the Payment Options bar) you will see "Have a coupon? | Have a gift card?". Click on the Have a coupon link. That will bring up a box where you can enter the coupon code and click Redeem.
Hope this helps!