Hi ahr19,

Sorry - I don't use Mac. You might be better off posting in the Mac OSX section
http://www.gnutellaforums.com/general-mac-osx-support/

You don't say which Java you're using...I don't know which version's best for OSX...you could use the forum search to check that out, or see if this helps.
Setting up Java 1.5

When you set up port forwarding, did you make sure to set up a static IP address first? You'll need to. Check out these instructions for your router, there's a link to static IP info there. Just go through those instructions & double check your setup.
Port Forwarding for the Linksys WRT54GS

You don't have to forward the whole range of ports, just choose one (64049 is one that heaps of people use, but others are fine). Ports in that higher range are recommended because they're sometimes the ones not being blocked by ISPs. I've always used the default port myself.