make the directory that you want to mount it to. You should have a /mount or /mnt directory. I would make the folder in this directory, something like /mount/usbdrive. Then, in command line:
mount -t auto /dev/sda1 /mount/usbdrive
this is the command mount, -t auto tells the filesystem of mounted device, /dev/sda1 is the device - this may be something different on your system, if say you have a SCSI hard disk. sda1 is Scsi Drive A partition 1. And /mount/usbdrive is where in the filesystem it is mounted to.
remember to check man mount for more help and options