Couple of simple things..
1. Is the camera in the cradle when you plug it into the USB port? (without the camera, the computer will not recognise the cradle)
2. Have you turned the camera on, once the cradle is plugged into the UDB port? (it needs to be switched on to be recognised and for the transfer of images!, use the small button on top rather than the slide down cover)
If the above two are ok, then maybe try a different USB port on your computer.