Hm, first try to get rid of the auto connect on start up. If neccessary, delete your ~/.xMule directory to do this but before you do this, make a copy of ~/.xMule/server.met to ~/server.met, for example.
Now restart xMule. This should make a new ~/.xMule directory. then copy your old ~/server.met back into the ~/.xMule directory. Restart xMule once more but from the command line using the command xmule. You should now have a server list in the server window.
If you have a firewall (iptables) running, make sure the xMule ports are open. The needed ports are shown in your xMule -> Preferences -> Connection Window.
If your ports are open, right click on a server (Not those with a the most users) and select "Connect to this server". Check the log messages and report back.