If you're eating at the Poly, then that's where you'd park. If you plan on going to the MK and you're staying offsite, I can't imagine them letting you park for free in the MK parking area. It IS WDW, so I suppose it's possible, but if it were me, I'd just go straight to the guardshack at the Poly, tell them you have an ADR(they'll ask your name and look it up) and park there.