Thanks for your help everyone. We are getting a taxi to the Marriott and then found out we can rent a car from Hertz right at the hotel. Then the hotel has a free shuttle to the Long Island RR.
Make sure that you study the LIRR timetable. The train station that you will be shuttled to is "Hempstead" You will have to travel West to Jamaica Station and get a train back (East) to Great Neck. Good Luck
From the KP Parents Page:
Travel by Rail
From Pennsylvania Station (34th Street at 7th Avenue, New York City), take the Port Washington branch of the Long Island Railroad to the Great Neck Station (about 35 minutes). Get off at the Great Neck Station. The "N58" Long Island Bus Service bus usually departs shortly after the train arrives, bound for the Academy (current fare is $2.00; exact change is required). The "N58" may be boarded from a location on the North side of the Station (you must take the pedestrian bridge over the tracks and cross the driveway adjacent to the ticketing/waiting area). Taxis are also available, if you prefer to travel by cab.