Note: ataidle options and rc startup settings
      have changed in version 2.

Add the following lines to /etc/rc.conf to enable ataidle:

ataidle_enable (bool): set to NO by default.
         Set to YES to enable ataidle.
ataidle_devices: list of devices on which to run ataidle
ataidle_adX: parameters to pass to ataidle(8)

Example:
ataidle_enable="YES"
ataidle_devices="ad0"
ataidle_ad0="-I 60 -S 120 -A 127 -P 254"