Hey there, Emily! It looks like you are only one sleep away from your trip to the Most Magical Place on Earth during the most wonderful time of the year. What a holly, jolly holiday!
I will keep this short and sweet so you do not become the White Rabbit and be late for a very important date. 
Disney dining plans have to be added to your vacation package before your check-in day for your 
Disney Resorts Collection stay. That means today!
If you purchased a 
vacation package (hotel stay and theme park tickets), you should be able to easily add a dining plan online. Go to "
My Plans," scroll down until you find your resort reservation and select "Change Reservation." You should be able to follow the prompts to add the 
Disney Dining Plan or 
Disney Quick-Service Dining Plan. If you have any challenges with this, please feel free to call 
Disney Resort Hotel Reservations. A friendly Cast Member should be able to help.
If you have an Annual Pass or other passes and purchased a resort-only reservation, you need to call the resort reservation line and speak with a Cast Member about the possibility of changing your reservation to a ticketless package. As an Annual Passholder, I had to call, and they were able to help with this. It could not be arranged online. 
Emily, I hope you can get a dining plan added and have safe travels to 
Walt Disney World Resort tomorrow. If we can ever help again, the 
planDisney panelists in Wonderland are here to help you on your adventures!
See 'Ya Real Soon!
Jana