If you are using a router and Windows XP Professional:

Make Sure UPnP is working:

Start
Run
services.msc
OK

Look in the list for Universal Plug and Play
Double-click
If started do nothing-

If not started start the service
On the drop down list choose Automatic
Click OK

If using a router with a GUI Web Interface (Linksys, DLink, etc)

Add Limewire to Port Forwarding (Choose a Port)

If using a firewall, Norton Internet Security, Norton Internet Worm Protection, and the like.

Make sure your firewall or antivirus software is not blocking the port.

Good Luck!