Download the dedicated server pack for NS. You may want a latter version than 2.01. The dedi pack gives you version 2, any later versions are applied server side, so your clients still use the ns 2 client.
Ticking the "dedi server" box on NS3 obviously runs an ns3 dedi server.

Just making sure.
NB:
Note that "listenserver.cfg" is for a NON-DEDICATED server, whilst "server.cfg" is for the dedicated server.
However, using the Dedi server pack is different, as it installs to a totaly different NS directory.
I have only used the server features within the client NS, so I can't comment more than that. HTH though.