Multiplayer is impossible?

[Aargh!]

So I gave away a few copies of this game because it seemed like a decent multiplayer game, but now we can't even get multiplayer to work. I've disabled my firewall, forwarded my ports... but nothing. I can see the created game on the server list but when I try to join nothing happens, and when I host the other guy can't join me either.

This basically means the game is complete ♥♥♥♥ to me, because single player is boring.