Frankfurt has good public transportation.
The subway, tram lines and bus run from 6:00 am through 1:00 am. Connections are best looked up on the RMV website . The Frankfurt public transport relies on the "honour system" ). You are expected to buy the ticket before you board the train. For buying a simple ticket there are "hotkeys" on the machine for "inner city" and "airport". A single trip within the inner city costs €2,50 per person. The trip from the airport into the city costs €4,10.
The "Nachtbus" (night bus) runs every 30 minutes from midnight to four in the morning. The central hub for the Nachtbus is the Konstablerwache where all lines meet up at the same time to allow for switching. The "n1" and "n2" lines connect the venue to the Konstablerwache.
Taxi fares are somewhat expensive, at €1,65/€1,75 per km, plus €2,75/€3,25 fee for day/night. The transfer from the airport to the venue costs about €20, to the train station about €30. Cabs are generally safe in Frankfurt except that the driver may try to drive a longer route if they notice you're not from around the town.
Route planning: http://www.rmv.de
Travel information: http://wikitravel.org/en/Frankfurt