the engine air filter

Here's a link. It's not on the right but left facing the engine and you don't just unscrew the case. http://mbworld.org/forums/new-m-class-w166/527242-changing-ml350-w166-engine-air-filter-3-5l-v6-gas-engine.html

Facing the engine, filter cartridge is located on top, right, front of engine. Unscrew casing.