The underground line U2 directly connects the Airport with the main railway station (Central Station) and the city centre. Alternatively you can take the bus line 32 to Nürnberg Thon or 33 to Nürnberg Buch. The night bus N12 ensures a connection on Friday nights, Saturday nights and the nights before public holidays. Get a map of the surrounding area and find all stops/stations.
Take the U2. After arriving at the main station, change to U1 train (direction: Fürth Hardhöhe). Leave the train at "Lorenzkirche" or "Weißer Turm". Price level A.
Change underground lines at main station ("Hauptbahnhof"). The U1 line (to "Langwasser Süd") will bring you to the trade fair in about 20 minutes. Price level A.