You must purchase a package to be able to add the Dining Plan.
To be considered a package, you must purchase at least a 1 day ticket for everyone in the room.
You can upgrade the days on the tickets at a later time.
Also, there is a time frame for adding the DP... by a certain number of days before your arrival, sorry, can't remember the exact number of days. (I mention this because it looks like you will visiting soon.)
Booking a package for an arrival date within 45 days, must be paid in full.
If you book room only, then add tix and DP, I think it then becomes a package, and they usually won't just transfer your room-only deposit. They may have to cancel, then refund and start new.
I would try splitting the total onto 2 cards.