Porte Maillot is in the 17th arrondissement, (NW Paris). It is at a large traffic circle about a km. beyond the Arc de Triomphe, following the line of the Champs Elysées. It is mostly easily found on a map by looking at the far northeast corner of the Bois de Boulogne.

There is a metro stop there. A colleague from Milan that I met in Paris last week, flew in on RyanAir for the sum of #1 roundtrip, plus a few taxes and fees. She took the shuttle you describe from the airport to Porte Maillot, then the metro to her hotel.

Joel in Chicago