The power steering reservoir in a 2002 Mercedes-Benz ML320 is located
under the hood, typically on the driver's side (left side in the US), near the firewall. It's usually a small, cylindrical or rectangular container with a cap. It might be somewhat hidden, so you might need to look closely and possibly use a flashlight. Look for a reservoir with markings indicating its function; it may say "Power Steering" or have a steering wheel symbol on the cap.
It's always a good idea to consult your owner's manual for a precise location and diagram.
