From Medan there is a local bus no air-con about every hour for 22,000 rupiah Oct 08 if you pay on the bus. The journey takes 4-6 hours, depending on the traffic condition.
From Medan RAJA Taxi +62 0617 366 555 will pick you up at the airport. Scheduled runs in an Avanza 5 person car will cost you 80,000 IDR per person Dec 08. Alternatively, you can book the whole car for 400,000 IDR.