In order to get the dining plan you must book a package thru Disney that includes at least 1 day of park passes for every member of your family. A $200 deposit would be due at the time of booking and the balance is due 45 days before the date of arrival.
The only exception to this is if you are staying at a DVC resort on points. If that is the case, then you would have to pay for your dining plan upon check-in at the resort.