If you have received a discount code, or coupon code; you will enter it during the checkout process.
First, you will need to fill out your contact email and address information, then choose, "continue to next step". If you prefer to pay via paypal, you can still choose to use this method on the next page.
On the next page, you will see a field to enter your discount.
Enter the code, click, "apply discount" and your order total should be updated.