To check if you have the motion sensor, just go into the "about this mac" item in the apple menu on the top left of the screen, click on the more info button.
In the left column of the new window make sure the top item "hardware" is selected, in the main frame on the right hand side of this window it should mention the motion sensor in the last 3 lines of the text describing the hardware setup.
If not, then u dont have a sensor, and that would be ur problem.