I never really went into the deep details of cron myself, but I think this may help.
You can edit the crontab of a specific user by using the crontab -e command as that user. (note; there IS a way to edit someone else's crontab without logging in as that person, This is usefull for if you have to edit a 'virtual' user like 'daemon' 's crontab. I'm doing this from memory but I think the option -l username will do that)
For commands that need to be executed repeatedly (e.g. hourly, daily or weekly), use crontab, which has the following options:
Install filename as your crontab file.
Edit your crontab file. (Note, by default this opens the editor 'ed', you can change that by setting the EDITOR to your favourite editor)
Show your crontab file.
Remove your crontab file.
As for your second question... I don't know.. sorry..
For more info see man crontab
I've never used Mandrake before, so if this info does not apply to mdk, please ignore it