LUN needs to be zoned or attached directly to Backup Exec server and not to another server
Tape Library also needs to be zoned or attached to Backup Exec srever
Data then moves across SAN from LUN to Backup Exec server and then back across SAN to tape library.
If LUN is attached/zoned to a different server (because the data inside the LUN is managed and owned by that server) then you will have to make this server into the Backup Exec server and attach the tape library to this server.
Please read the blog I linked ealier as it explains all the scenarios and it is possible you currently have a configuration that cannot do any of the SAN based scenarios - if this is the case then there is nothing we can do until you change your environmental configuration (which might involve investment in specific hardware or extra software )