The Event is at the Unperfekthaus in Essen. Essen has no commercial airport, but all airports in the region have trains to go to Essen Hbf (main railway station) from where it is walking distance or you just take a taxi for around 5 EUR.
Airports that are fine would be:
DUS – Düsseldorf
CGN – Cologne Bonn
DTM – Dortmund
also Ryanair flys to NRN – Weeze – from there is no train but a shuttle bus (or if travelling with more ppl a rental would be an option)
For intercontinental flights it could be an option to not take a connecting flight to DUS, instead go on the train or rental in FRA – Frankfurt or AMS – Amsterdam