Charlie
The NDMP option only works with NetApp filers that speak that protocol (see the compatibility list). For Windows-based NAS, you should use the Remote agent for Windows systems as it is supported and these boxes don't implement NDMP.
An NDMP backup just launches the backup command to the NAS server, and the NAS servers performs the action to a local device. Backup Exec does not write any data, it simply sends the order. It's the only way you can protect your NAS data to a tape device.
Hope that helps, regards