Have a look through the user guide, it explains how to enable UPnP within your modem. You also need that option checked within LW's tools > options > advanced > firewall config. It's no use having that UPnP option checked if you don't have it enabled within the router.
http://subscriber.communications.sie...UG_English.pdf

If enabling UPnP doesn't help, try port forwarding. For advice, check the site below. There are step-by-step instructions. Don't forget to set up a static IP first, else port forwarding won't work. In this case, make sure that the manual port forward option's checked within the LW firewall config.
http://www.portforward.com/english/r...0/Limewire.htm

I use AVG & it hasn't stuffed anything up for me. Are you using the free version?