Hello, my name is Rusty. If the walking belt is off-center, first remove the key and UNPLUG THE POWER CORD. If the walking belt has shifted to the left, use the hex key to turn the left idler roller screw clockwise 1/2 of a turn; if the walking belt has shifted to the right, turn the left idler roller screw counterclockwise 1/2 of a turn. Be careful not to overtighten the walking belt. Then, plug in the power cord, insert the key, and run the treadmill for a few minutes. Repeat until the walking belt is centered.
That screw is at the very back end of the treadmill, to the side of the belt.
Hmm, does it seem to move the belt at all? Even a little bit? Is the tension on the hex screw loose or tight?