The gas tank on a Mitsubishi Outlander is located on the rear driver’s side of the vehicle. It’s not hard to find if you go to the rear of the vehicle. That said, you’re probably talking about the gas filler door release, which is a bit more tricky.
The Mitsubishi Outlander gas release lever is located on the floorboard on the driver’s side near the door jamb. This is far different from the dash-mounted buttons on most modern cars, so you’re certainly not alone in having trouble with this.