Если поедете сразу с аэропорта, вот описание, как туда добраться::
"You can get from Medan airport to Lake Toba entirely by public transport if you wish. First, you must go from the airport to Amplas bus terminal. Walk through the car park and go out the main car exit of the airport, cross the road and turn left after the petrol station, and then turn right (about a ten-fifteen min walk altogether). There, you can catch a yellow public bus, number 64, to Amplas Terminal for Rp 3,000. From Amplas terminal, you can get a public bus to Parapat for Rp 22,000. So you can get from Medan airport to Parapat for a total of Rp 25,000 (September 2012)." Wikitravel.
Ну а дальше, в Парапате, выходите на пристани Tiga Raja, откуда на пароме за полчаса доберётесь до Тук-тука (населённого пункта, где обычно останавливаются туристы).