I have two linux boxes connected to each other via a hub, I want to test file sharing between these two boxes using Napshare. These pc's are connected to the Internet and use private addresses. Will Napshare work in this scenario, if yes what am i supposed to get it working? Any comments and thoughts?

See the comments at the bottom of the settings file, you need to allow your private addresses for the "ip_block" and then simply connect one to the other using their IP address and port number, make sure each pc has its firewall off (for testing) or firewall pass through for the port you chose.
If you don't want it to connect to the outside world, remove any "urlcache" entries also and deny all others that are not private addresses.
I think I remember the README file having something about private LAN sharing.