If you have a discount code, you can enter the code on the last step of the payment process. There are 3 steps when paying by credit card and 4 when paying by Paypal.

The last page of the payment process will have a section at the top called Your Order.   At the top of the page you will see the discount code section. Note: This field will be available no matter which country you are from and/or you are paying with Paypal.

Enter your discount code and click Update Coupon to apply the discount and update the amount.  You will see a confirmation and the new total.