First, thanks Rebeltinkle.
The Polynesian sits between the main parking area for the Magic Kingdom and the transportation hub.
Mass is held in the area used for the luaus, from the parking area on the right side of the main hall and keep bearing right as you head between the buildings. From the park you would have to take either th momrail or the ferry back to the transportation hub and then head over to the resort... Or you could catch one of the boats that goes directly from the park to the resort and come up to the luau area from the docks.
Either way, I'd plan a good 1/2 hour from the time you get on a boat or monorail until you get to the luau area. Also, the Masses are often crowded, so to get a seat up close you'll want to get there early! (The congregation sits at the tables used for the luau, so if you end up a long way from the stage area it can be difficult to hear the priest.)