The most convenient airport is Oslo Airport (OSL). Upon arrival take the FB2 bus to Oslo Bus Terminal (40 minutes). From here transfer to the NX3 bus to Strömstad Station (2 hours). The festival site is an 8 minute walk north from the bus station.
Strömstad Station is accessible from most cities in Sweden and Norway but will most likely require at least one transfer along the way. The festival site is an 8 minute walk north from the station.