Exit the airport and go right, following the way that cars are going out. When you reach the road at the intersection, go left towards a bigger intersection and turn right there (it is signposted for Villach and Wien). This connects you to the Salzburg ring road. If you walk about for about 500 m (passing a tunnel going down and back up) you reach a roundabout next to a petrol station. This is not the best spot, but it may work out well. Cars are able to come back even if they miss you on first pass.