Namur is a city on a human scale, you can easily walk through it and go to the festival's venues concentrated in the old city.
The buses of Namur are taking you anywhere. More info on the prices:
There are one direct trains to Namur every hour from Brussels Airport and two trains an hour with a connection via Brussels North station.
Some of the Ryanair flights land in Charleroi (Brussels South) Airport. It’s 30 minutes from Namur with a taxi (46€) or you can a bus to Charleroi (20') then a train for Namur (30').