To enable the script you will have to create a symlink in the plugins-enabled folder to the plugin in the plugins-available folder like it is done for the other plugins. Afterwards you edit the file /usr/local/ispconfig/server/lib/config.inc.php and add the line:
$conf['services']['awstats'] = true;
Then the plugin should work. The author of the plugin may correct me if I missed an additional step as I did not tested the plugin yet. An of xourse the awstats package from your linux distribution has to be installed.