Punta Cana->Santo Domingo->Autopista Duarte->Hwy 12 to Jarabacoa->through town (don't bear left at the ayuntiamento), turn right at the stoplight, turn left at the bottom of the hill->to Manabao->rt. to La Cienaga.
It's a 3-day hike at minimum. I'd suggest getting in touch with Rancho Biaguate to arrange an excursion to PD. You really can't just go and take the excursion. It takes planning, animals and a guide.