For this first you need to determine the SCSI ID of the physical tape device as connected to the ESX :
Log into the ESX console and check the value of the same under : /proc/scsi/"tape drive"/ and you will find the ID : value listed above.
Ensure that in the properties page, the Device status is set to connect / connect at power on (both).
This should add the device to the VM, I e the backup software should recognize the tape.
Please let me know if this helps.