Eindhoven Airport is very close to the festival site, though Amsterdam Schiphol is larger and served by more destinations.
The number 400 and 401 buses depart from Eindhoven airport four times per hour, and take 20 minutes to reach Eindhoven station. From there, Intercity trains also depart four times per hour to Roermond. The journey takes about 30 minutes, with the total cost of the journey being roughly €15.
You can also get a direct train to Roermond from Amsterdam Schiphol.
You can also reach Roermond via rail, however, many services will run via Eindhoven or Dusseldorf and may require a bus for the last part of the journey.