The faster and easiest way from POP (airport) is to take a cab. It'll cost you $200 one way.
If you're not arriving in POP but staying in Puerto Plata, then there is a bus that leaves at 6:30am every day to Samana: Transportes Papagayo. I believe you can take it right in front of the hospital. You'll need to get off in Sanchez and take a local bus or cab to Las Terrenas.