I have recenty configured OTRS for our small helpdesk. I have to say it's great and has revolutionised our support profile

The only issue I have at the moment is that any email sent to our postmaster address is not automatically fetched by OTRS. I have to go to Admin -> Postmaster Mail Accounts -> Fetch mail.
I know this can be setup to automatically to fetch every 5 minutes for example but I am unsure of how to do this with Cron.
If anyone could help I would be greatly appreciative.
Thanks
Tom