To apply a promo code for discounted ticket prices:
1.) From the event details page that the promo is for, type the promo code into the Promo Code field (right side of screen) and then click "submit". This will reload the web page and apply that code. See the picture below:
2.) Click Buy Now / Select your seats for that discounted event. You will see the Promo with discounted ticket price.
3.) Click the View Ticket Details to review your seat(s), then click the Add To Cart button to purchase. Enjoy the show!