If the connection test fails but you can still connect to the networks, this means that at some point there's something that denies incoming connections. Either a firewall or a router (or both) which need to be told that it must accept incoming connections on the port used by Shareaza.
So the only suggestion that comes to my mind is that you should make sure that your modem doesn't have a buil-in router feature, which is very common nowadays. If you're modem is shown on
http://www.2wire.com/index.php?p=1 there may well be a reason to suspect that it indeed has a built-in router since the page reads "All in one intelligent manageable device: modem, router, wireless access point, firewall and more." You may need to read its documentation or user manual, and mentioning precisely its model here may help.