There are a couple of places you can enter the code.
For mobile users, click on "CART" in the top right of the screen. There is a box called "Coupon" - enter the code here.
Alternatively, also for mobile users, you can add the code when you start checkout. Open the link "Show order summary" and you will see a box called "Discount code". Enter the code here.
If you're using a computer or tablet then the boxes should be easy to find either in the shopping cart or when you start the checkout. The boxes are the same as for mobile. Enter your code into one of these boxes, it doesn't matter which.
If you still can't see where to put the code then please contact firstname.lastname@example.org and we'll guide you.