I was there just recently, but can't remember the name of the Metro station for sure, but I think it was Porte Maillot, definitely on La Defense line though.
It is just down from the Arc De Triumphe and the car park they use to get the bus is right opposite a large high rise hotel called the something Concorde.
Make sure you get there at least 3 hours before your plane is going.
It is always a bit unorganised getting the bus back as everyone wanders around looking confused trying to find the right bus, the drivers don't speak English.