ssh into your account and type "locate jadmin.cron.php". It will give you the full path. I think the full path should be something like "/home/yourusername/yourdomain/components/com_jadmin/jadmin.cron.php".
Once you have the full path, you can set up crontab job like the following
*/5 * * * php FULLPATH/jadmin.cron.php >/dev/null 2>&1
You can discard the username when you set up the crontab job because each user has its own contab job settings. By default, it will run the owners jobs only.