You can only use LW 5 on an intel machine made in past couple of years. LW site will auto detect your system I believe. One way to check if your system is compatible is go to Java preferences, if you do not have Java 1.6 (Java 6) listed then your computer is not able to use LW 5. Setting up Java for Leopard (see 2nd image)

Make sure LW is set as an exception in OSX's firewall.

What kind of wireless system do you have? Brand & model. This might be causing a firewall effect, most wireless systems tend to.

Change your gnutella.net file over to a new one.