I am desperately trying to start an online server (non-PW) for me and my friend to play together on. LAN is not an option. However, even with the Client Extension, he is unable to join my game.
I had him enter my actual IP x.x.x.x.x:5121 instead of the one it provides, however this has not worked either.

I've read some stuff about changing my firewall to allow for specific ports, however I've found no further information in this regard, and don't know how to make those edits.

Please help.
NWN2 only means something to me if I can play it with my friend.

What you need to do has to do with "port forwarding". I don't know how to do it, but I know that's how you directly connect with people in any multiplayer game. All I can suggest is looking up that term.