I'm not able to start the cron daemon automatically on Raspbmc. I tried to configure an upstart and an init script but had no luck.
You can set it via the settings as per user9428's answer, and also activate by setting the following to true:
<setting id="sys.service.cron" value="true" />
in the following file:
Replace .xbmc with .kodi if you're on Kodi already.