I must warn you this router was a "lemon" that Linksys released. Normally their devices are very good. That range of models had firmware problems Linksys never bothered to fix. Port forwarding is definitely the safest approach. But be sure you did it correctly. First step MUST be setting up a static ip (for OSX). If you do that step later then the process won't work & are better off deleting that port forward rule & repeating correctly. 2nd & 3rd steps are port forwarding (PF) the device & setting LW up for PF setting. I'd also tend to recommend a separate port forwarding for each of TCP & UDP.

Also see the link called Linksys WRT54G - Fixed my problem, Followed these steps and it works! under UNnP section -> Linksys about 1/4 way down the page here Connection problems? Check here first! (click on link). But don't do that unless you are confident. You'd be best keeping a note of Firmware text just in case you did attempt it so you could undo it if you struck problems. Not for the simple minded to attempt because it's risky.

Are you sure you disabled your OSX firewall? Disabling or setting it up for LW's access can be seen here OSX Firewall settings for Tiger.

Try setting up a Test account & try opening LW from there & see if it connects.

Also, go to your Connections window & tell us if you are seeing any attempts at connecting to an ultrapeer. And if any do actually connect. For Basic you will get a maximum of 3 & Pro 5, unless you are acting in Ultrapeer mode.

leandropucci also try setting up a test account to see if you can open from there & check your connections window.

In sample image below, you see I already have an account set up called Test. And one of the test accounts is active at that time. Easy to swap between accounts using login window at top left of screen. (click to see larger image samples)