The best way to get to ENS Cachan is to take the express train from Paris (RER B) and to stop at the Bagneux station (not Cachan). It will take you 20-25 minutes from central Paris.
Then walk to the main entrance of ENS Cachan (about 10 minutes).
Upon reaching the main entrance, pass the fence and make a left to the Pavillon de Jardins. The Salle des Conférences is on the first (ground) floor.