You can catch the guagua from Nagua to Rio San Juan at Avenida Maria Trinidad Sanchez. They'll take you to the stop at the junction on the same autopista at the traffic light. Catch the next guagua from rio San Juan to Pto plata there . 300 pesos each one (600 total) The last one from Nagua is at 6 pm, but if you get to rio san juan at 7pm there is more than a good chance there will not be one there waiting for you to take you to Pto Plata and if it's the last one you sometimes have to pay double. So try to leave nagua by 5pm the latest. The earliest from nagua is around 6.30 am