The quickest way is to rent a van, but it will be very expensive. It takes 9 hours or so by train or bus. Nantong Bus Station should have one bus at early morning to Tunxi District of Huangshan City. Suggest you get to the bus station and ask for the details one day in advance. If it is available to buy the ticket in advance, you can just buy the ticket.
Or you can make a transfer at Nanjing. Nantong-Nanjing buses are leaving every 20-30 minutes. Nanjing Zhongyangmen Bus Station and Nanjing East Bus Station all have buses to Tunxi or huangshan Scenic area. The timetake is: Nanjing Zhongyangmen - Tunxi at 7:00, 19:30, Zhongyangmen - Huangshan Mountain area at 8:00. Nanjing East Bus Station - Huangshan Mountain area at 13:40 to Tunxi at 19:00. I think the 13:40 bus is a better choice. Or you can take train from Nanjing to Huangshan which will be a bit more comfortable than bus. Huangshan Railway Station and Tunxi are all about 1 hours bus drive to the scenic resort of Huangshan Mountain.

hi there we are planing our tour for next year september 2011 in china,from beijing,xian and shanghai,but i would like to make another stop and see mt.huangshan!!!
so can we fly from xian to there or we have to go from shanghai???
and we are traveling with our kids(10-7yo)just for one night on the mountain,see the sunrise the next day, a bit of hiking and leave for the next city...
can you plan our itinery if possible pls
kind regards

Xian do not have air flight to Huangshan. If you want to go the mountain from Xian, fly to Hefei first and then fly to Huangshan. Or it takes 3-4 hours by bus to Huangshan. Then it will be dark at night. Find a place to stay, see the mountains the next day. If you go from Shanghai, Shanghai South Bus Station has buses to Huangshan for 5-6 hours ride. Take the earliest bus and arrive at Huangshan at noon. Then you have enough time to see the mountain. Another cheaper option is: take night bus from Xian to Hefei and arrive in the city in the morning. Then take bus to Huangshan 3 hours.