After attaching your usb device, run
and look at the few last lines. Your device should be recognized as sdX1 (X is a letter, usually X=a if you don't have any scsi disk or other usb attached). Then run:
mount /dev/sda1 /mnt/usb
Change sda1 to whatever your usb devide is named and note that you must create first the directory /mnt/usb (if it doesn't exist).