In Webmin 1.430 on Darwin 2008 the error message "The Cron jobs directory /etc/crontab does not exist. Maybe the module's configuration is incorrect, or Cron is not installed?" appears on "Scheduled Cron Jobs" module.
When the information on Wikipedia  is correct , cron (and several other similar scheduling services) support has been dropped from OS X since Leopard (10.5).
The new OS X replacement is named launchd.
Feature request is to disable cron, at, etc. for OS X relevant versions and enable a launchd compatible alternative.