Login and go to your Meal Plan Preferences page. At the bottom of the page you will see "Current Meal Plan Balance", as shown below.
Click "View my balance details"
You will then click "Add a gift card", where you will see a text box appear. Enter in your gift card code inside the text box and click save.
Once you click "Save", you can then click done. Your Current Meal Plan Balance will reflect the gift card you just added.