It's a bit tricky to find the e-certificate if you don't know where to look.

From the My Account page, click the "Upcoming Reservations" link. Unused certificates are found there.

Some of the reservation agents refer to this e-certificate as a 43J.

I know about when my 43J will arrive (plus or minus a few days). I usually make my award reservation before the 43J shows up in my account, and hold the reservation as if I am going to "pay" with Marriott Reward points. When the 43J pops in, I call back and change my reservation so that I am using the 43J rather than the points.

Why do that? Let's say you expect your 43J to arrive by May 31. But you want to book a reward night in late June. If you wait until early June to call and reserve the room, it is possible that there might not be any rooms still available for awards. (This isn't a problem if you're planning an award stay in Podunk, but it can be a problem if you've visiting a popular tourist destination.)