To take advantage of one of the offers displayed in Drop Down Deals, simply click it within the window. Your browser will be redirected (to recover the necessary codes), and then redirected to page from which you started.
When your starting page re-opens, the Drop Down Deals window will now contain the details for the coupon you selected. Click the newly-added coupon code, and it will be copied to your clipboard. At the checkout screen, paste the code into the provided coupon code field in the checkout page of the site you're on.