> M0n0wall detect just one interface, and after select it ask me for
> another one (the WAN inteface). This is a problem. I've tried using a
> second network card not connected to any network, but m0n0wall can't
> detect it. So, I connected to second network card to the same networt
> than the first one. And the problem continues...
> 

Simply set up the LAN interface and connect it to your hub.

Choose another unused IP for the WAN (within a private address range, 
say 10.0.0.1/8). There is no need to have it actually connected to a 
physical network.

If the initial configuration console does not detects the WAN interface, 
  try to reach the WebGUI from your LAN and click on the "assign" link.

That should work...

-- Vincent