In order to be able to purchase one of the dining plans, you have to purchase a WDW package. A WDW package is a room at one fo the Disney owned resorts with MYW tickets. If you're staying off property then you have to pay OOP for all of your meals. Dining Plans are only an option for the WDW packages.
The exception to the package are those staying at DVC (Disney Vacation Club). They can purchase the dining plan and do not need park tickets. But, as you assumed, you still have to be staying on property.