Thanks for the question, Sherri!
If you booked your vacation directly with Disney, you can typically add the dining plan as late as 72 hours (3 days) before the beginning of your arrival day. To be safe (and not have to count the hours), I'd do it no later than 4 days before you arrive. If you used a different travel agency, you should check to see if their policy is the same.
Please be aware that you can only add the Disney Dining plan to Package reservations (hotel, plus at least one day of park tickets). Some exceptions to this rule are made for Annual Passholders (call Disney, or refer to the Annual Passholder site, for details).
For more information about the Dining Plan, please click
HERE.
Hope that helps!