The Dining Plan is an add on to a package. Packages usually include tickets. I say usually, because there have been some reports that annual passholders have been allowed to book a package that includes the DP without buying a ticket. Maybe that is where the confusion comes in.
That said, if that package were available for your dates, you would have had to book that way to begin with - it's not something you can change at check in. Don't upgrade unless that is the best deal on admission for you.