Hi,

In <YourInstallation>/extensions/ezsi/doc/documentation.html file, there is no reference to a necessary option to place on virtualhost.
The option is "Options +Includes" (referred in http://httpd.apache.org/docs/2.0/howto/ssi.html )

As example of my virtualhost:

    <Directory /var/www/apache2php53/ezp50/>
        Options FollowSymLinks +Includes
        order allow,deny
        allow from all
        Options +Includes
    </Directory>