You will be given the option to redeem any certificates you have on-file once you advance to the payment page. Once you identify the hotel you want to book and click "View Rates", you will then see a tab near the top of the page that says, "Redeem Points". You then proceed by clicking Continue. At this point the next page below your contact information you should see the option to "Attach e-certificate(s) for your reservation."
Hope this is helpful.