On the "Make a Reservation" page for the Hoop De Doo there's a blue link at the bottom of the page that says "Use my dining plan to book a prepaid reservation." Click on that link and input your reservation info and you should be able to use your dining credits. The total for the credit card payment will be $0.00, although you'll have to input the cc info for the no show/cancellation guarantee. If you're still having problems, you can call the dining res line.
Edited to add: There are only certain show times and seating categories that can be booked using the dining plan. According to the dining plan pdf it looks like it's Cat 2 & 3 for all shows and Cat 1 only for the last show of the evening.