In the shopping cart, click the plus sign where you see "If you have a partner or coupon code." Then just copy/paste your code into the gift card box and hit Submit.

You're welcome Oh, and if you ever find that you need to apply more than one gift card (like when you have partial amounts leftover on one), you can apply the first code, then copy/paste the next one over the first and submit again. The cart tallys them as you go.