I was afraid it might be a commercial theme. I don't have access to that one.
What would be the purpose of overriding post revisions
Some designers like the "uncluttered" approach to their user interface. If this designer happens to incorporate additional options in the theme related admin interface, you may find an option to turn it back on from there. Otherwise you might search the theme files for something that intentionally hides the check-box. In this case it might be something along the lines of
remove_meta_box('revisionsdiv') for example (I'm no coder, so that's just a guess).
Failing that, and because the evidence now suggests that it seems to be a commercial theme related issue, it's something you will need to address through the theme developers support channel.
 Ha! You're way ahead of me. I just spotted your support request in the Modulo support FAQ. Hang in there... sometimes it takes a while depending on how attentive the author is. Good luck!