Take the train to Utrecht Central station, here you can take the bus to "de Bilt/Zeist". These busses leave from the Jaarbuszijde, platform C4.
Take bus: 50, 52, 53 or 74 and get off the bus at the stop "Tunneltje de Bilt".
Walk through the underpass and keep left. After about two minutes you will see the white building of ORMIT.