iCloud switch is disabled
The iCloud syncing switch in Mental Case is disabled when iCloud is not available to the app. There can be two reasons for this:
1) You are not logged into an iCloud account in the Settings
2) You are logged into iCloud, but you have not turned on syncing of 'Documents & Data' in the list of things to put in iCloud.
If you make sure you are logged in, and Documents & Data is on, then when you relaunch Mental Case, you should be able to switch on iCloud syncing.
Studies for macOS and iOS
Mental Case for Mac OS X
Mental Case for iOS