Plug the usb drive in and type dmesg in a console. It should give you a device like sda1 or uba1.
Otherwise you can have it plugged in on bootup and do cat /etc/fstab to see if it was correctly identified.
If you have a line for it in fstab it can be mounted with : sudo mount /mnt/mountpoint_from_fstab
Even if it's not in fstab you can mount it with:
sudo su
mkdir /pen
mount /dev/device_from_dmesg /pen -t vfat