I'd recommend to park somewhere near the train station (guess it's quite easy to find) and like 1.5 km from the square where the gig will take place. Furthermore, it's really wasy to get there on foot from the station, here's a little map:
just "imagine" getting out from the station, you have simply to turn left and follow the "main road" till you see a castle, than go round the castle till you get to the festival entrance
it's really easy. Btw, Ferrara is a lovely city, the city center not big at all, good for a walk around before getting in line to enter.