From Paris CHATELET-LES-HALLES : Take the RER line B direction SAINT-RÉMY-LÈS-CHEVREUSE. Stop at MASSY-PALAISEAU and take the bus line 91-06 direction Saint-Quentin (Mondays to Fridays). Stop at "Le Moulon".
You can also stop at the station "Le Guichet". Then take the bus line APTR 269.002 "Le Guichet-Saclay" (Mondays to Fridays) on the square located sligthly to the north of the RER station. Stop at "Supélec"