Hi Connie,
Assuming that you have booked a
Magic Your Way Vacation Package, you are indeed able to add a
Disney Dining Plan but this must be done at least 48 hours prior to your arrival at Walt Disney World.
Whilst my family absolutely love the Disney Dining Plan in certainly isn't for anyone and to help you to decide if it will benefit your family during your visit I would firstly suggest sitting down and working out how many quick-service and table-service meals you are likely to have each day of your trip. Then head on over to the menu section of the unofficial website Allears.net where you will be able to get a rough idea of how much each meal is likely to cost.
Once you have done this you should be able to work out if it is likely to be more expensive to pay out of pocket for your meals or to add a Disney Dining Plan.
Either way, I would highly recommend making advance dining reservations for all table-service restaurants as soon as possible as you will be visiting at the busiest time of year and many popular restaurants do get booked up quickly. Advance dining reservations can be made up to 180 days in advance on the Walt Disney World website or by calling (407) 939-3463.
Wishing you a magical trip filled with memories which will last a lifetime!