Under the payment step on the registration step wizard (setup->registration->registration setup wizard), you will see a box for entering discount code information.

Discount codes get applied to fee groups. You can select a maximum number of players to apply the discount code, as well as the code to use for the discount. The start and expiration fields specify the time period that the discount code will be valid.

After setting up discount code, a code entry box will display on the fee summary step of registration.