one:
driving in DR is nor exactly easy and safe. rent a 4WD for some extra comfort (big holes in the road). it will also pay off when you get to 27 waterfalls - the road goes through cane fields and after the rain it is not passable in a normal car (sometimes not even in a jeep, tours use big trucks).
two:
going out from SD stay on main road, it will take you across the country and through santiago. follow signs for puerto plata. about 30 minutes after you leave santiago the road will take you right. another 30 minutes of driving, just before luperon you will see 27 waterfalls on your right. it is not very big and quite easy to miss if the cane is high so keep your eyes open. few minutes drive through the cane you will get to the big building that is the base for all trips.
have fun.