To be able to use a discount code, you must be logged in to your Topsnus account - otherwise the code will not work. If you don't have a account, you need to create one first.
When you have added to your cart what you want, click on the link "My Cart" in the top right. On the cart page there is a field for entering the code. Then click "Apply coupon" and continue with the checkout process.
IMPORTANT! If you abort the checkout after you have clicked on "Proceed to checkout", you can no longer use the code. So don't do this unless you are ready to place an order. Also, you can only use the code once.