How do I hook into Pro Site's cron job

I'm trying to hook into psts_process_stats to send my own expiration emails. I have a function that send emails at different periods of their trial depending on what day they are at. Here's what I'm trying to do:

add_action( 'psts_process_stats', array(&$this, 'sendExpirationEmails') );

I made a plugin for this. Can you tell me why this doesn't work? Thanks.