If you run cronjob using PHP command, and get no PHP errors, it means cronjob might be executed without any problems and there is a problem with your script.
Your cronjob should look like this:
"php -f /home/username/public_html/file.php"
We would recommend you contacting the developers of the script that you are trying to run.
Additionally you can find useful information here: http://www.google.com/search?q=cronjob+does+not+work