Okay....the correct answer is that you can take either the RESORT monorail or the boat launch from MK to get to GF. If you take the monorail, make sure it is the resort monorail. It will stop at CR-TTC-Poly - THEN Grand Floridian which is your stop. You an also take tht boat which goes from MK directly to GF, then ot teh Poly and then back to MK. The monorail runs more frequently and travels faster, but since GF is the fourth stop - the boat maybe faster if you time it correctly. (i.e. if you see a boat approaching the dock, probably try the boat. If you just miss the boat, go up the monorail ramp.)
1900 Park Fare is just inside the lobby of GF, onthe ground floor. You'll need to take the elevator to the first floor then head toward the resort entrance. If you are entering teh building's main entrance (under the monorail station) then 1900PF will be on the left.
I'd allow about 25 minutes from the ENTRANCE of MK to get to your ADR. You are supposed to arrive a few minutes before your ADR, and that should be about the right amount of time.