Hey hojongsoo! You can check for Rewards redemption availability on your online account. Once you've logged in, go to "My Account", then "Find & Reserve". Once you've filled in the information that it asks from you, you should see a "Use Rewards Points" button.
All available hotels will be listed there, along with their category numbers, the paid rate and the the number of points required. Since you are redeeming a free night that are valid at certain hotel categories, it may be best to limit your selection to hotels in you certificate's range.
After you click "View Rates" you can make your selection from the choices provided under the tab, "Redeem Points". You'll then confirm your room preferences and click "Select" to proceed.
On the "Review Reservation Details" page, you'll see:
The message "You have unused e-certificate(s) that match your reservation" will be displayed in the "Summary of Charges" section. Your unused promotional certificate will be listed, and it will be automatically selected for redemption. Your stay must be completed on or before that date.
After reviewing your reservation details, please click the "Book Now" button to complete your reservation. The electronic free night certificate will be attached to your reservation, so you will only need to provide your account number upon check-in. You should also receive a reservation confirmation via email.
If you prefer, you can call our Reservations office for assistance. You may locate your area's Reservation office online at:
Please note that neither points nor money may be applied to upgrade a promotional free night certificate for a higher hotel category.
Hope this helps! Let me know if you have trouble or any additional questions
As lindseyh noted you will want to choose the "Use Rewards Point" option when booking your stay. You can also call Marriott reservations and they will be able to attach the certificate to your reservation.
Just be sure to tell them which certificate you want to use if you have more than one. With different expiration dates being possible, you want to use the ones that expire soonest first (I know - captain obvious, but it's something easily overlooked and the system is not automatically set up to do that).