I just replaced a tub/shower valve with a moen 2 handle valve.

I cannot seam to turn off the water to the shower head. When I push the lever down on the tub spout, the shower will not shut off. Thewater pressure slows down slightly but does not shut off at the shower head.

Just my luck, that the valve is now behind a new fiberglass wall panel.

Could I have installed the valve upside down?

The diverter on the tub spout does not turn the shower head on or off. The water naturally flows down to the tub spout unless you block the pipe, which is what you are doing when you pull up the knob. That forces the water to flow up to the shower head. Remove the tub spout to see if it is defective. Remove the valve cartridge to see if there is something blocking the bottom outlet port.