By Media Server I presume you are referring to the PC with the DAT drive
Correct
I've already mapped drive Z: to the NAS box but Backup Exec doesn't recognise
You cannot backup a mapped drive, you need to back the share on the remote computer
Again I presume if I install the RemoteAgent on the PC it shd work, is that right?
If you can install the RemoteAgent on the NAS, then yes, this would probably solve your connection problem. The problem is that most NASes are a cut down OS that won't accept software installs. If you cannot install the Remote Agnet, then you need to create a UserDefinedShare using either the NAS's name or IP address