When you enable mailslots, BE will not display them when you click on the library. For example, if you have 48 slots in your library and you enable 3 mailslot, then you would only see 45 slots in BE. Check that this is so with your installation. If you have enabled 3 mailslots and BE is showing more or less than 45 slots, then do the following
1) Delete the library from BE
2) Stop the media server and the library
3) Start the tape library
4) Start the media server
5) Ensure that the library and tape drive is recognised and functioning correctly in the Windows Device Manager
6) Add the library to BE
7) Check that the number of slots in BE is what you expect.
You should check the configuration of your library to make sure how many mailslots are enabled. If you are not sure how to check your library configuration, you should talk to your IBM rep. In your case, there could be no mailslot or 1 - 3 mailslot enabled. If the mailslots are not enabled, then they would be treated as normal slots and would be visible in BE.