After you've made your travel selection, below the cancellation policy there there is a field to input your promo code. Expand the field and type in your code. Click submit to have the promo code applied.
Why don't I see a promo code field?
Promo codes are only valid for pre-paid travel or lodging. If the the booking you are viewing is post-pay, it will not be eligible for use with a promo code.