The Fondation Louis Vuitton is at 8 avenue du Mahatma Gandhi in the Bois de Boulogne, in Paris' 16th Arrondissement.
The museum has its own shuttle - a small electric bus that departs from the Place Charles de Gaulle at Avenue de Friedland, near the entrance to the Metro. Shuttles come every 15 minutes or so and drop you right in front of the museum - the easiest way to go without having to do some walking from public transportation. The shuttle costs 1 euro, about $1.25, each way.
For the Metro, take Line 1 to Les Sablons station, and take the Fondation Louis Vuitton exit. Fondation Louis Vuitton is a 10- to 15-minute walk from the station.
Another idea, if you're so inclined, is to take advantage of the public bike system, Velib'. Pick one up at the nearest rental rack from your starting point; there's a Velib' station right in front of the museum (en.velib.paris.fr).
Noon to 7 p.m. Monday, Wednesday and Thursday, until 11 p.m. Friday, and 11 a.m. to 8 p.m. Saturday and Sunday; closed Tuesdays.