The Promotion Code report contains the list of people who used a promotion code to register for the event. 

The report can be accessed by clicking "My Accounts" in the left menu, then clicking the "Reports" button for a particular event.

You can run the report, then look in column C to see what promotion code was used.

You can create a custom report by clicking the "Customise" button next to a certain report type.

To learn how to customise a report, click here: