Hello Moustafa,
The Mailslot is something that you need to configure using the Web Interface or the Front panel of your device. Backup exec would not be able to see it i.e. it would be invisible to Backup Exec.
As far as locking and unlocking the device is concerned, you can do it by referring to http://www.symantec.com/docs/TECH33500
Note: A tape would go to the mailslot when you run the Export job : http://www.symantec.com/docs/TECH34389
As far as the error mentioned, I'd suggest that you refer to the following links or best contact HP for further help:
http://h30499.www3.hp.com/t5/Tape-Backup-Small-and-Medium/MSL2024-Close-Mailslot-amp-Robotic-Failure-9B-3B/td-p/5836977#.Uyqii_mSyAg
http://h30499.www3.hp.com/t5/Tape-Libraries-and-Drives/MSL2024-Robotic-failure-9B-3B/td-p/5756733#.Uyqii_mSyAg