Go to Users -> Manage Users and "Edit" the user. User Account Type needs to be "Shell account - allows SFTP/FTP plus ssh access."
*/20 * * * * /usr/bin/curl http://www.mydomain.com/wp-content/plugins/wp-o-matic/cron.php?code=07d39f95
That is a crontab entry, not a command!
The error message explains it all. The Cron Jobs panel is the user-friendly version ("cron for dummies") of executing commands at certain times. And in this version you enter the command in one field and schedule the times using a different field. And then what the panel will do is use that information to add a crontab entry. See DreamHost Wiki - Setting up cron via the DH Control Panel
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