Directions to LIX. A video showing how to drive here.
For some hotel information in Orsay, in Palaiseau, and in Paris, see this page maintained by Catuscia Palamidessi.
There is a bus (91-10: Massy, Plateau du Moulon, Orly) which circulates between Ecole Polytechnique and Massy-Palaiseau RER station. A time table a nd bus route is available.
If you are interested in renting an apartment (for a week, month, etc) near Polytechnique, my assistant has some suggestions and can help you find them. Science Accueil has as its primary focus the helping of visiting scientist to the Saclay - Orsay - Palaiseau area with finding accommodations. Some visitors have reported good experiences with the following two organizations that rent appartments in Paris: http://www.parisaddress.com/ and http://www.apartments-in-paris.com/.
Some advice recently offered (Sept 2009) for finding an apartment in Paris. The text is in Italian (try this for Google translation to English).