"Once you have your item(s) in the cart, go click "Proceed to Checkout" and then click "Redemption Code?" under the "Payment Methods" section"
Thanks - I tried this, but it didn't work for me - told me code was invalid. It may be because I am from outside the USA. However, at the stage that I had entered the code I hadn't changed the shipping destination (but I guess it may know from cookies?).