You can drive right to the Boardwalk resort and park there- just tell the guard at the gate that you have dinner reservations, and he will issue you a 3 hour pass. There is self parking in a lot that is a 5 minute walk from the lobby, or you can pay $10 to valet park- just go all the way up to the lobby entrance and you'll see the valet parking stand.
If you don't want to drive, you can always take Disney transportation. Take a bus to MGM, then you can take a boat to the Boardwalk, or you can even walk- it is a nice 10 minute walk from MGM.