The bus is Transportes Papagayo and it goes to Samana, not to Las Terrenas. They'll leave you in Sanchez where you need to take a cab. They leave Puerto Plata Hospital around 6:45am once a day. They stop in front of Janet's supermarket in Cabarete, but will stop anywhere if you signal them. I was told they go through Cabarete at about 7:15am. I think on the way back from Samana, it goes through Sanchez at around 2:15pm, but you need to verify this.
The cost of the ride was RD$400. Since my information is from two years ago, call them up before your departure date: 809/749-6415 (they don't speak English) to confirm the time and cost again.