launchd and croncron is deprecated under Mac OS X 10.4 and 10.5; it is replaced by launchd. You can schedule tasks with launchd by making a .plist, putting the plist in ~/Library/LaunchAgents/, and running something like
launchd load ~/Library/LaunchAgents/my-task.plist. Cron is still present on the system, though. If you still want to use cron, just set up your crontab as usual with
crontab -eand cron will automagically be turned on (launchd will load com.vix.cron, which tells it to check the crontab). a couple more details on launchd and cron, see also crontab.