They usually show up under Account Activity at the bottom under all the actual activity in a separate section with the header Unused Certificates.
I've made the 2 stays, but the second still hasn't credited after over a week so no cert for me yet (I have a missing stay request in though).
Maybe this will help you know where to look.
Using the certificate is as simple as booking a room on points (up to a category 5 property). Then just "attach" the certificate to the reservation. Here's an example how to do that Re: Unused Rewards Certificate.