http://www.tunngle.net/index.php?l=en#l=en&pg=downloads
Go here, get everyone to download this program.
Start up Tunngle, join the simulation/Silent Hunter 4 channel.
Then, start SH4 and continue from there.
Go into the multiplayer button. And choose LAN.
Then, it should be pretty straight forward from there.
Let me know how it works.
-Aur0ra145