When you get given a promotional code, head on over to the pricing page, choose the package that you would like (or the one that relates to the promo code) and click 'purchase'.
At the checkout, click the button asking if you have a promo code and enter it exactly as we've typed it. Click apply and you will see the discount already added.
Note that your account will auto-renew at the usual rate at the end of the promotion. For details on stopping this check out this page.