Kellie,
Since the only bus leaves at 6 AM, your only options to get there the same night are to rent a car and drive (probably not a good idea unless you have driven in the DR before) or hire a taxi to LT. You may want to consider this if you have already reserved your room in Las terrenas. The cost of a hotel room in Santiago plus cab fare to the hotel and then to the bus station, bus fare and transport from Sanchez to LT will probably be almost as much, or maybe more, depending on how much you spend for the hotel in Santiago.