I'll assume your running a kernel that supports both USB and SCSI Disk (sd_mod). Once you plug in the USB drive, wait 10 seconds and type dmesg, the output should list a new devices at /dev/sd_ and it will likely be /dev/sda, but that is up to the OS/udev to decide. Once you know that your usb drive is /dev/sda, then you can mount it with mount -t vfat /dev/sda /mnt/usb assuming you have /mnt/usb created. You could mount it to any empty path you choose.
If your uncertain that the correct kernel modules are installed, post the output of lsmod and lspci so that we can talk you through the steps of enabling the correct modules.