Carlos,
Are you sure crond is running?
I'm pretty sure it is and I see this on the logs
Sep 23 10 pam_unix(cron): session opened for user www-data by (uid=0)
Sep 23 10 (www-data) CMD ( /usr/bin/php /var/www/api/cron.php)
Sep 23 10 pam_unix(cron): session closed for user www-data
Also, shouldn't I be able to run it by hand or is that not how it works? I don't know much about php.