From the user manual. Sorry I'm not too familiar with iDesk but I'll help you the best I can.
Of course you need to set up an email address first, then what you want I believe this is found under Tickets- Email Processing
To fetch the incoming tickets, select the accounts you want to then click button “Pop emails”.
This process may take some times to be completed. Except the unbanneds, all incoming
emails will be fetched and new tickets will be generated.
This pop processing can also be done automatically with cronjobs.
The following cron configuration will run email processing every minute.
Code:
* * * * * GET http://www.mydomain.com/helpdesk/cron/email_processing.php > /dev/null