You need to download the Installtion files for the RALUs and RAMS packages and then locally extract/install the programs on your MAC (Leopard) systems
Note: Snow Leopard is not supported (yet) and you can't do a push install from the media server, this only works when the remote system is also running a Windows based operating system.
You should be able to get the download for 12.5 from:
https://www4.symantec.com/Vrt/offer?a_id=51792
Also your Backup Exec (winodws based) media server will need the serial number for the Mac agent installing.