On a computer, once your cart is validated, the summary of your order will be displayed on the right side of your screen, throughout the ordering process until payment. At the top of this summary will be the "discount code" field in which you will have to enter your promotional code and validate it.
On mobile devices, the "discount code" field will appear at checkout, after you've validated your shopping cart, entered your personal information, and then selected your delivery option. You'll need to enter your promotional code and click the arrow to the right of the field to apply it before proceeding to checkout.