as root in a terminal type:
lsusb
see what shows up, it should show all usb devices, if it doesnt show any, type:
lsmod
and see if the usb kernel modules are loaded
to check to see if what macdude suggested worked, type:
ls /mnt/usb
the directory /mnt/usb has to exist, but I believe it will kick out an error if not when using the mount command macdude gave you