I don't know what command you use in CRON task.
If WGET, then
http://www.sunnysihota.com/zen/cgi-b...d=yes&ping=yes
If php-cli, then
<path_to_home_directory>/zen/cgi-bin/sitemapxml.php rebuild=yes ping=yes
About <path_to_home_directory> ask your hosting. It depends on your hosting.