There are two opportunities to enter your promotion code.
In your Shopping Bag, select the 'Do you have a promotion code' drop down. Enter your promotion code as it appears and select apply.
Or, at Step 3 Payment Details at the checkout, select 'Do you have a promotion code' drop down. Enter your promotion code in the promo code box and select apply.
Please note, a promotion code is not a Gift Card, and should not be entered into these fields.