Iñaki,
if you configure in your Netview "Options -> Topology/Status and
Polling..." as follows:
      Enable Polling and Discovery Settings     ON
      ...
      Discover New Nodes      OFF   => will discover the nodes listed in
your "seed_file"
      or
      Discover New Nodes      ON    => will discover all accesible nodes in
your network

your also have to specify your seed_file for the netmon. That can be done
through the "Server Setup"

You can find more information about using the seed file in the Chapter 7 of
the "Installation and Configuration" Netview Manual.
I hope this helps.

Regards,
      José Rodríguez