The bus you need is labelled "2 - Campus". Both number and name are relevant. The combination guarantees that you make the trip in the right direction. I have put the detailed timetable here. The first pages are for the return trip. Thursday noon, the bottom half of page 7 is relevant. Friday morning, it's page 6 that matters.
This bus stops near all your hotels. Some will enter at "Leuven Station". there is a stop (if I remember well) across the street at Theater hotel. People staying at Ibis will enter it at Fochplein. It then goes down Naamsestraat, where there is a stop nearby the ICCAM conference site, convenient for those staying at hotel New Dampshire.
The cheapest way is to buy a ticket in advance at a newspaper shop or at the company shop at the station. The name of the company is "De Lijn", by the way. You can buy a ticket on the bus. That will cost 2 euro and you need to give the driver the correct amount. Coins of 1 and 2 euro are extremely useful!
One question remains: where to leave the bus? You have to leave at the final stop, "Heverlee Campus Arenberg III" at the timetable. The bus stops right in front of building 200A, the dept. of Computer Science, where you are expected on the top floor. The campus map is at the bottom of this page, but I don't think you need this. Elevator and stairs are located in the middle of the building.