Check with David's. They can verify whether it's there or not and tell you how to find it if it's not obvious. As I recall, the dining plan isn't on the first page of info; you have to go to the details page to see it. I just reserved a dining plan for someone renting some of our points, and I was a little worried when it didn't show up on the reservation summary. But then I clicked the details view and it was there.
In general David's will check the reservation after the member completes the dining plan purchase to make sure it's there. If it's really not on the reservation, perhaps the member has canceled the reservation accidentally, in which case David's should get on them to fix things.