Difficulty setting up a cron job

Hi,
I'm having difficulty in setting up cron task for retrieving blogs. The task itself runs fine when run the command in a web browser.

http://mydomainname/index.php?option=com_easyblog&task=cron

My service provider is saying I need to provide the script name as a full server path, but I'm thinking doing this is just going to run a PHP file and not pull config data from Joomla. They are saying I need to use something like:
/usr/bin/php /home/sites/mydomainname/public_html/components/com_easyblog/classes/mailbox.php

Can you give me some advice please?

Regards

Stephen