After looking at the map, as I thought, GOLFGUY's directions are right on except substitute Sabaneta de Yasica instead of Gaspar Hernandez as the name of the town where the road which goes through Moca and Jamao intersects with the North Coast highway.
There are basically 3 main routes from Santiago to the North Coast, and there are strong opinions on this board as to which is best. I feel very strongly all 3 ways!
If your destination is Cabarete (or points East), the route described here is best.
If you are going to Sosua or the East side of Puerto Plata (Playa Dorada), take the Carretera Turistica. (Yes, I know the road is not as good, but it's closer.)
If you are headed to the West side of Puerto Plata (Cofresi or Costumbar) take the Autopista Duarte toward Navarete to the North Coast highway.