I'm using this to monitor another plesk 7 system that I have. It always show the "WEB Server status" of the slave server as down eventhough it is running. Any idea?

Check firewall rules on each server, that's likely the problem.

Does anyone know which ports are the Plesk system using to monitor Slave servers?

Just standard 8443, but you need to allow 2 way (in and out traffic) communication via this port rather than just the standard 1 way.

I have tried disabling APF on both servers, but it still show the same.

Try "service iptables stop" to ensure that the rules are clear then try again. Are you certain the password is correct?

Same thing. =) The password is working correctly 'coz it shows all other services status as available, except webserver and tomcat.

Sorry I didn't read your initial post correctly. I thought you meant it was showing all services as down. Does Apache show as up on the remote server?