As DW (lecras) said above, the waitress said that they didn't accept it. I had to insist that she go and check.
We had problems with the DDE card quite often. At several places the staff had to go and check with a Manager because they didn't know what it was and some confused it with the dining plan. However the most common problem was that they didn't think the discount also applied to alchohol.
It saved us a lot of money, so it's well worth having, but at times I had to be very insistant to get it accepted, almost to the point were I was made to feel that I was being difficult.