With the plugin Cronjobs overview you can edit and execute existing cronjobs. Edit means you can edit crontab, not the content of cronjob.
The creation of new cronjobs in backend of CONTENIDO is not possible because of security reason. They must exist in the file system.
New cronjobs can be uploaded via FTP or SSH to server in contenido/cronjobs folder.
After copy you can edit crontab file and add your cronjob.
To configure crontab see: http://www.adminschoice.com/crontab-quick-reference/ or have a look at the Cronjob page.