A:
The light turns on just for a few seconds when you turn on the mouse or when it has a low battery level, if the light doen't come up at all, make shure that all the drivers are properly installed, try installing the logitech software, you can find it in here:
http://support.logitech.com/product/wireless-combo-mk520#download
Download and install the setpoint installer, i hope that helps you