Tadachi is located on the Chūō Main Line. The best (and cheapest) way to get here from Nagoya is to take the train - 90 minutes. You can use the JR Aozora Day Pass for ¥2500 to cover all your train transport for the day.
To get to the mountains, you have to first go through the village, up the mountain to the highest car park and the starting point of the hiking trail - 6km from the train station. There is a map near the train station and once you reach the highest car park, there are printed maps available.
Hike up the trail along the many waterfalls.