So that your promo code got activated, you have to:
Sign into the service, go to "My Account" and enter the code on the Dashboard page in the field "Promo codes". After the code is applied, your discount would be activated automatically, and you will be able to pay the subscription with a discount.
Be attentive! Each promo code has its own validation period which indicates the latest date the code can be activated. After the period expires, promo code is not valid.