Please do as following to redeem the coupon you've received:
Select the Fixel Algorithms product from the drop down list.
Wrote / Paste the coupon you received in the coupon text box.
Click on the Acquire the Product! button.
You'll then be transfered to page for purchase with the updated price according to the coupon code. Make sure you insert a valid email as you'll get the download link of the product via email.