IF you have started the CIV IV game, and logging into Multiplayer within the game, and once you have put in your credentials, but only stalls, and/or gives you a prompt that you are unable to join, perhaps its your NAT-Firewall you are connected to.
http://forums.civfanatics.com/showthread.php?t=146735
I would suggest reading this forum, and see if it might resolve your problem. Better yet, I would suggest directly connecting your computer to your modem, and see if that resolves your issue. If you have already connected your computer to your modem directly, then I can only suggest trying another computer, or reinstalling the game.
Perhaps there is an Application firewall on your computer preventing you from connecting, such as ZoneAlarm, or WindowsXP firewall. You may want to (as a troubleshooting step) disable these features. If and when you have determined if its the problem, then immediately restart your application firewall, and if its the problem, search FAQ's in order to resolve the issue.
I have personally ran into the problem, even if my port forwarding rules were correctly setup, and restarting my modem and computer seemed to resolve the problem. Of course you could go more technical and perform a TCPDUMP (Ethereal) trace, and really dig into what the problem may be.
Cheers;