The discount code can be placed in the Personal Details section of the checkout process, which comes after the Date Selection and Ticket selection.
The option to add a Promo/Voucher Code can be found on the right, underneath the event details. The customer needs to first input their details, then click on that option.
The box will then appear to input the code, which can be used once per customer. Then they need to press Apply, and continue the checkout process.