what always will work: look into your /dev directory with
this will show you all disks sda* sdb* and so on. In most cases /dev/sda1 /dev/sda2 ... are the partitions on your harddisk. So /dev/sdb1 or /dev/sdc1 should be the usb device. Mount as root or with sudo:
sudo mount /dev/sdb1 /mnt
if sdb1 is your USB-device. Be aware that the mountpoint, /mnt in this case must exist and should be an empty directory before mounting.