If it doesn't show up in Disk Management, and your OS is Windows XP or higher (Vista, 7) and you use the USB connection (not the 1394 in Windows) something is wrong hardware-wise.
Things you can try:
- does another external disk work on your PC
- can this disk be seen by disc management at another PC?
- does Linux (boot from a Linux Live CD) see it?
Back to the shop, let them show it works with them and, if not, get your money back.