If a coupon code has been generated for you, you can enter it at checkout at the screen where you enter your details. The location is just above the "Place order now" button.
In order for some coupon codes to validate you must be logged in to your cart when placing your order. Please refer to the attached for the login icon.
Note: If you don't already have an account, you can go ahead and register now and then use the coupon code.