It is officially the Terminal Atlantico Norte, but nobody calls it that. Tell the taxi driver that you're going to the Terminal San Carlos. Yes, you should take a taxi. It's another bus terminal in a dicey neighborhood. The company is called Autotransportes MEPE. Website is in Spanish only:

www.mepecr.com

The bus will probably say SIXAOLA on the front. That's its final destination.

It takes about four hours to get to Cahuita. There's a rest stop at about the three-hour mark.

Jeff,
We have a family of 5 and another of 4 that will be going to Costa Rica mid June. We are flying into SJO then going to Arenal for 3 days, then Manuel Antonio for 4 days. Taking everything into consideration(gas cost, road conditions, road signs, etc), would you recommend us renting a car or arranging for a driver for each leg of our trip?
I know most of the activities we will be doing in Arenal have someone to pick us up at our hotel. When we get to Manuel Antonio we will be staying in a villa and will need to get food, go the beach, etc and I am worried about getting around there. Any suggestions would be appreciated. Thanks!

Taxis are pretty easy to come by in the Manuel Antonio area, and most things are pretty close together. Where we stayed we could walk to several restaurants and a grocery store easily. For us at least, a car would have been a waste in MA. Where are you staying? Arenal is much more spread out; if you want to go into town for meals or are staying outside of town, you probably want to consider renting a car because taxi fare can really add up there. Our most expensive taxi ride in MA was $5; in the Arenal area it was $25.