We have come across a few instances where, after enabling the new security options in 8.1.1, the new Security Details menu option is not visible on the ribbon.
If this happens, there is a simple fix. Simply remove the app from the affected site, and then re-add the app. The Security Details menu option will now be visible.
Note that as we store all configuration information in the configuration database, removing and re-adding the app will not cause any issues, and will retain all previous configuration.