After you've created a Voucher Code Promotion, you want to actually see what the code(s) are. From the Home screen in the back office (where you can view all current Promotions), hover over Voucher Code Promotion you create and then select View Codes.
This screen will display the single code (i.e FRIDAY) or unique codes (i.e YXZ123, ABC456) associated with the promotion as well which of your customers have redeemed the code and when. You can download a full list of the codes and and customer redemptions using the Download .CSV button.