It sounds like you have a tape drive, not a library and it needs to be configured in DP as a stand alone drive.
In order to get the tapes rotating, you can setup the Pool to have a Loose or Strict Policy (I advise a Loose Policy) and set up the backup to eject media after use - This can be found on the Drive Settings, Advance options, Eject Media after session check box.
When the backup is finished, the tape is ejected and you can remove it to the vault/fire safe. When you want to re-use it, just insert it into the drive and DP should just re-use it as long as the protection has expired.
HTH
Regards
Mark