There's no 'force IP' option any more, it's been replaced with the option to manually port forward.

Read through this sticky
Connection problems? Check here first!
Make sure your Norton firewall's configured properly & that the XP firewall is off. You may have an internal firewall within your modem/router, there's a lot of info in the sticky. If your router supports UPnP then enable it & tick also within LW's options > advanced > firewall config. See if that fixes the firewall problem. If not, you might have to manually port forward.

What are your details (excuse the repeat questions)

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