Promotional codes need to be added to your user account prior to making an offer to the lounge.
Visit the wallet tab in your profile, and add the promotional code to the box shown below, and then click 'Apply':
If successful, the coupon will then show:
The promotional code will also show on the payment details box prior to submitting your lounge offer:
Note however that promo codes can only be used towards successful bids that are paid directly with LoungePair.
For example, if you receive a voucher that shows payment at the lounge, there will be no coupon deduction.
This is regardless of what is shown on the payment details box.