The promotion code needs to be entered in exactly as it was given to you, without any spaces. Type or copy and paste the code into the 'Discount Codes' box.
Once you've entered the code, click on 'Apply Code' and it will add the code to your order. When the code has been applied to your order, you'll see the promotional value deducted and the total of your order will change to reflect the code.
The code will only apply as per the terms of the promotion, and to relevant items in your order that are included in the promotion. If you are still having issues with your code, ensure the products you want to purchase are eligible.
To make the text box appear, click on the arrow at the end of 'Discount Codes ' as below: