Have you tried sharing out the resource and backing it up that way?
If it's an entire drive, check its properties and share it out. You won't be able to give it a name - it takes the drive letter.
Grant access to the ARCserve admin account, then see if you can see it in your network tree in the backup window. You may have to close and re-open the backup window to refresh the view.
If the resource is just a folder and files, just share it out. It'll show up under the server in your backup window, and as long as you've granted access to your admin account you should be fine...
HTH...
Jake Cutter
IT Operations: Always outnumbered, always outgunned...