Coupons are used for offline marketing so you need also an offline processor that will check the validity of the coupon and register the sale.
To register an offline coupon sale, you need to go to Offline Sale in the main menu in the merchant panel. Here you can enter the coupon code to check its validity:
If the coupon is valid, new form will appear so you can create a sale commission. The commission will appear as a new sale commission in the Transactions > Commissions.