It's actually quite simple. When you exit the park follow the signs to go east on Osceola Parkway.
Follow the signs that say Magic Kingdom and you should end up on World Dr. headed north.
Stay on World Dr. until you get to the Magic Kingdom toll plaza (stay to the right side of the plaza). Once you are through, there is a road that leads off on the right side - this takes you to the resorts. I believe it's the second left turn that you want to take. From there, follow the signs to the Polynesian (it will be on your right).
That should be it.