There are two companies running airport busses from the Trondheim airport Værnes. Both go to the city centre, but take slightly different routes. The FIRST City Center stop of the bus “Værnesekspressen” is the Conference Venue Hotel, Clarion Hotel & Congress. Both busses stop at the Central Station, and Søndre gate, the stops closest to most other hotels in the city centre. The fare is around NOK 180 one way if tickets are bought on the bus. You can pay with Visa or MasterCard. Passengers get a discount if buying a round trip, or online: Flybussen or Værnesekspressen.
For a map of the bus stops and iBOL2019 Conference sites can be found here: Map of Trondheim.