By Express Boat
Maybe the most comfortable way to travel between Bergen and Rosendal, the express boat Rødne Fjord Cruise (http://www.rodne.no) have a couple of departures every weekday and one every saturday and sunday. The boat is a bit more expensive than the bus, but gives a traveller a much better view. If the view is not nice enough there is also free a wireless internet connection on board. The boat also have scheduled stops at Bergen airport on most trips.
By bus
There are buses going from Bergen to Rosendal. As this would require change of bus in most cases, and timetables on the web is mostly in Norwegian only, it may be easiest to contact the information phone 177 or a tourist information office.
By car
Rosendal is about 2-2,5 hours drive from Bergen, and about 2 hours drive from Haugesund. From Bergen you'll have to take one or two car ferries, depending on route.