If this is a drive that is new/ has nothing on it and you want to add it to the system follow this howto
If the drive is already partitioned and has a file system on it and you just want to mount it on a temporary bases, find the root partition of the drive in mention like so. As #root in a terminal type fdisk -l
this will list all drives your kernel sees. Lets say it comes up sdc3 then do this.
commands as root.
mount /dev/sdc3 /mnt/temp
Then after your done and want to un-mount the drive make sure your not within the /mnt/temp directory then type the command.
umount /dev/sdc3 /mnt/temp