If I recall, you can do it one of two ways.
Firstly, call the reservations line and let them know you have this night. They'll see it in your account and can book the room.
Secondly, if you click on the box "Use Marriott Reward Points", I think it will use your free night when you book (not certain on this).
I find that calling customer service is the easiest and quickest way to use the free night. One short phone call as opposed to winding your way through the maze of finding how to do it online.....a no brainer for me to dial the number! They have always been very efficient with the phone request!