If using the "PayPal Express" option during checkout, it will have you log in to your PayPal first, then returns you to the Zabaware site to confirm contact information. Before it complete the transaction it gives you the opportunity for the discount code then presents you with a final "quote" and then you have to click "confirm" to complete the transaction.
If using a credit card, during checkout, on the page before it asks for credit card info is a box labeled discount code. Enter the code there and 50% will be taken off of everything in your shopping cart.