Activate the Step Sensor from the Step Screen, it is the one right after the clock.

It shows Step in the bottom right corner of the screen

See a little person on the left side of the screen? 

If not, your step sensor is paused. If your pedometer ever stops counting steps this is the first thing to check.

Hold "MODE" button down until you see the little person return on the left side of the screen

Congratulations, your Step Sensor is now Activated . Proceed to setting your clock.