You must start at the homepage for this to work. Once there, go to the bottom of the page and click Express Deals as pictured.
You will now see a slightly altered format but the same options at the top of the page. To use a coupon you must be in the Express Deals part of the website.
Choose your hotel and decide on your preferences if asked.
You have your opportunity on the next page next to the summary of charges. Look for a small link that says "Apply Coupon".