Have you checked Windows Device Manager?
The default Windows Device Driver (Unknown Medium Changer) for the tape library is normally sufficient.
If tape drivers are loaded, they will appear towards the bottom as Tape Drives.
If they don't show up, you need to verify hardware connectivity.
FC or SCSI?
If FC, through a switch or direct to server hba?
It may even be related to the hba in the server (FC or SCSI) - Have you checked hardware compatibility with library and tape vendor?
Once all is seen in Device Manager, then run scan command to verify that devices are responding to scsi-commands.