Update. I had another frustrating conversation, during which they then wanted a faxed CC statement to prove that I'd actually dined. They don't need this because their system wouldn't have credited the 20% discount unless I had dined, and they've closed their own website so it's their fault if they can't see the information on their screen.
I did convince them they didn't need this information, by giving them the reference numbers for the dine and the 20% credit from my statement.
Now they say they'll credit me $50 on my next statement.
I'm waiting for a confirmation number for this process by email -- I'll believe it when I see it.
I had been really enthusiastic about this program before this happened -- but now, I'm about ready to scream. I will not be recommending Dish to anyone, but it's closed anyway so they probably don't care.
I'm just sorry for the people who answer their phones, and are probably told by their managers to make the customers run around in circles so that they get tired of pursuing the refund they are owed by Amex Dish. Poor employees -- but when I can't speak to a manager, there's no-one else to whom I can express my frustration.