I tried and tried to redeem my cert online, but gave up and called. The Marriott customer service person told me, "you can only redeem these free certs over the phone." I did try the above procedure and could not bring up the free certs.

One of the things I like about Marriott was the good phone customer service, but I sure got transferred around a lot, seems like you can't call just one number to handle any reservation issue and any point redemption issue.