Are you connected well (how many green connection bars)? If you're not connected, do this test (click on link)
http://www3.limewire.com:6346

If you're seeing the firewall icon, it's either a problem with a software firewall or an internal firewall within a modem or router. There's a lot of firewall information here
Connection problems? Check here first!
Make sure you're only using 1 software firewall & that it's configured for LW. Read through the info about modems/routers, there's info about UPnP & port forwarding.

What are your details?

OS(Win XP etc)
LW & Java version (help > about LW & you'll see)
Firewall you're using
Modem/router brand & model
ISP
Connection type (DSL etc)
Where you connect from (home, school etc)
Are you sharing a connection with other computers
Is this your first try with LW