Amsterdam Schiphol is the closest international airport. Direct trains run from the airport to Utrecht Centraal station every 15 minutes and take half an hour. The journey will cost you €9 for a single.
From Utrecht Centraal station, take the tram in the direction of IJsselstein or Nieuwegein, and get off at the Vasco da Gama Avenue stop. From there, it is a short walk to Park Transwijk.
Alternatively, a number 65 bus from the station towards Vianen will take you the 5 minute journey to the Utrecht, Europaplein station. Park Transwijk is just to the north of the stop.
It is not possible to park next to the festival site. The closest option is Transferium Westraven.
This is an 18+ event. You may be required to show ID upon entry.