mailman & ispconfig 3.0.4.3 installation problem

[Giuseppe]

Hi all,
i have some problem when install mailman on already existent ispconfig 3.0.4.3 installation.
I hope someone can help me.
My environment is:
Debian squeeze 6.0.4
ispconfig 3.0.4.3
ispconfig is installed on cp.mydomain.it on port 80

I had ispconfig already installed without mailman installation, but now i need mailman so i have do the follow:
apt-get install mailman
newlist mailman
added alias about mailman list in /etc/aliases
newaliases
/etc/init.d/postfix restart
/etc/init.d/mailman start
php -q update.php reconfiguring services

BUT:
after that up done when i go to http://cp.mydomain.it/mailman/listinfo i have:
not found The requested URL /mailman/listinfo was not found on this server.
if i go to: http://cp.zyzhsp.it/cgi-bin/mailman/listinfo i have:
Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, webmaster@localhost and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Please help me!
Thank you for your collaboration.

Giuseppe