The bus is ok for getting into Palermo on an evening, but the train is far better during the day. Traffic in Palermo during the day is horrendous, and the bus once took me two hours to get to 'Centrale'.
You will find the train station (at the Airport) as you leave the baggage collection area, just to the left and down the escalator. Its roughly the same price as the bus, but will get you to 'Centrale' train station in about 40 minutes.