Have you read the ReadMe file section on "Push"? Gives some good info.

In Napster>Config, do you have a reflected IP shown? You can ping others, but can you ping your machine from outside the LAN? Of course other Napster clients don't need to know your IP to work, and if you're behind a firewall running IP masq (hope you are) Napster should work with a reflected external IP like 65.XX.XX.XXX. but you should be able to ping your local 192.XXX.X.X IP. If not, maybe check your router's NAT config.

I use Napster from behind Bastille firewall running both IP masq and NAT with no problem from this machine, which doubles as a router/firewall for a small LAN. I don't even have to force the IP, so it'll work, ya just might have to play around some to find what's not setup quite right.