We just returned from WDW, and DAK. We asked at the ticket counter about the meal plus certificates, and was told that we could purchase them at any of the restaurants. We bought ours at Pizzafari on our way to the safari ride (before lunch and the lines were long) and then used them later at the Flame Tree.
They were a great value for us. We saved $4.00-$5.00 per adult and $3.00 per child using the certificates. I just wish some of the other parks would participate, too.