You host a game by choosing Play from the starting menu and then Multiplayer from the NWN menu. Actually, that's what you do if you are looking to host the game and play it as a player at the same time, which is typically what people are looking to do, so that the person hosting can join with a character and play the module with his friends. There are other ways to host a module as a server without playing at the same time or to host a module as a DM. What I describe below is for hosting a module as a player.
From the multiplayer screen, you start a module by choosing "New Internet/LAN Game" and picking the module that you want to play. On that same "Load Module" screen, you can select other options (such as a password to let people join as players, etc.). It's been a while since I looked at those options, but I seem to recall that most of the defaults were playable.
Typically, your friends will connect on port 5121 (that is something you can change with the "Advanced Options" button of that "Load Module" screen). To find your PC's actual IP address, just google "what is my ip address" and google will tell you your public IP address that your friends can use to get to your hosted game. (Note that your public IP isn't needed if you are hosting a LAN game, such as if you are playing with PCs all connected to the same router.) Then the other players can join by going to their multiplayer menu, choosing "Join Internet Game" and then clicking "Direct Connect". From there they can enter your public IP followed by a colon and then your port. They might also enter a player password, if you have set one. For instance, if I had left my port settings at the default 5121 and google told me that my public IP was 76.254.105.71, then my friends could join by entering 76.254.105.71:5121 as the Server Address in their Direct Connect screen, plus whatever password I have set for them, if I set one.
