what flavor of *nix are you using?
the mount command is used to "mount" local/remote file systems for use. since tape drives are basicaly recording devices, they do not actually contain a file system you can use, so they cannot be mounted. assuming that your kernel supports the device, a device file should be set up automatically for you once *nix detects it (ie. boot-up), on most linux boxes the first tape drive will show up as /dev/rmt0 (for r_emovable m_edia t_ape number=0) but different *nix's will have other conventions.
the standard *nix backup is done with tar (t_ape ar_chive), in the format:
"tar -cv yourdirectoryhere"
without the -f option we normally use, _on some systems_ tar will assume you want to write to the first tape device.
*these are very sketchy instructions without knowing exactly which *nix you are using!
hope this helps!