by Emma Richardson.
Try this - first run cron as your website user - for example, in linux from terminal
crontab -u www-data -e
Then add the command with cli as noted above
So it should read:
*/30**** /usr/bin/php /var/www/moodle/admin/cli/cron.php >/dev/null
It looks like you were directing to the website instead of the actual folder plus on 2.0 plus you do need the cli reference.