Where is the power steering fill tube on 2011 mariner

There is no fill tube, but a PS reservoir on the passenger side of the vehicle.

your vehicle has an electric power steering system ("EPS" or "EPAS"). These systems use an electric motor instead of a hydraulic system and therefore do not have any power steering fluid to leak. If you're having issues with the power steering, you should check to see if it was a recall item on your year vehicle. It was a recall item on the '08 model I own.