Hi, I found what I think you are looking for.
I was wondering the same because it wasn't until after I called the Marriott # they told me i had achieved some rewards.
Once logged in to Marriott Rewards click on "Promotion Central" under my account, then click "New Promotions for you"
This will take you to the screen showing all of the current promotions open and if you achieved one, you can then click on the achieved reward and book the reward from that screen.
Hope this helps.
On my account it doesn't have a certificate number. But when I was ready to use the certificate, I logged into my account and chose my hotel and dates and so on. At the bottom it asks if you are using your Marriott points - click yes. Then as you go through the next few steps, you will be able to choose if you want to use your certificate or points automatically. It took me a while to figure out how to use the certificates because I myself was looking for a number as well. After a few attempts and frustrations, I found out the ones you get for the promos and such do not have numbers. Hope this helps!