In order to do archive scheduling, MailSteward uses the system crontab. Installing Mountain Lion deletes the /etc/crontab file that the crontab system needs in order to function. The following steps will fix the problem:

1. Open a terminal window and become the OS X system root user, by entering the 'su' command and then entering your system root user password. If you have not already enabled your system root user, here is how to do it:
  • Choose Apple menu > System Preferences and click Accounts.
  • Open Accounts preferences
  • Click Login Options.
  • Click either Edit or Join in the Network Account Server section.
  • Click Open Directory Utility.
  • Click the lock icon to unlock it, and then enter your administrator name and password.
  • Choose Edit > Enable Root User, and then enter a root password in the Password and Verify fields.
2. Execute the following command on the command line: 
touch /etc/crontab
That's all there is to it. Once this is done, you can close the terminal window. MailSteward scheduling will now work.