Short answer:
setup cron (*nix) or taskscheduler (windows) to run cron.php every so often.
Long answer:
This is taken from the POP3/IMAP settings document regarding setting up polling located at: http://osticket.com/wiki/POP3/IMAP_Settings(http://osticket.com/wiki/POP3/IMAP_Settings)
Recurring tasks scheduler (Cron Job)
This is the most convenient method if your hosting provider allows scheduling recurring tasks via crontab on *nix or "Scheduled Tasks" on a Windows server. Please refer to your hosting manual or contact your hosting provider for instructions. The PHP CLI executable is called to run cron.php.
Add the following entry to cron file normally in /etc/crontab in *nix systems and adjust the time and paths accordingly.
*/5 * * * * nobody /path/to/php /path/to/api/cron.php
For windows users in "Scheduled Tasks" add "c:\php\bin\php.exe c:\website\osticket\api\cron.php"