Assuming the cradle was working before under Vista, and assuming you're running Win98 on a different computer, the problem is with that computer, either in the software or hardware.
Does the Win98 computer even have USB 2.0 ports, and are they enabled in the BIOS? What else is connected? Try disconnecting everything and make sure the cradle is connected to a port in the back. Does the computer do anything when the DMS is inserted in the cradle -- make a sound, anything?
It's possible that some previously installed USB drivers are causing a conflict with the cradle drivers. Check your installed programs and drivers, and uninstall as much as you can.
If nothing else works, the easiest way to solve the problem might be to install XP instead of Win98...