What brand router? The router needs to allow Port 6346 ("mapped"/"forwarded"), at least. Do other computers connect to the modem too?
Then there's the OSX firewall (System Preferences->Sharing->Firewall->Other->Gnutella/Limewire (6346)--or risk leaving it off altogether.
Then there's the Limewire Prefs->Advanced->Firewalls->Force IP [add the external static IP assigned by your ISP, not the internal IP of your Local Area Network].

[edit: the following is no longer relevant: If your ISP connects you with an IP that changes (DHCP--dynamic host connection protocol), you'll have to find a way to check what IP the outside world sees every time you connect. This is something that is difficult to describe from here--perhaps your ISP can help you to find what your external IP is and how often it changes. If you connect via DHCP, you're better off forgetting about using the "forced IP" setting. ]