How many cable are connected to your monitor? You should have three cables coming from it if you want the usb ports to work.
one is the power cable, one is the cable that is plugged into your video card, the last one should be plugged into a usb port on your comp. The third cable is the one that allows the usb ports on the side of the comp to work.
Of course this info is taken from my own experience with monitor setups that have usb ports on them.