Pretty much what you said. It's there however when i click to activate it just goes back to the previous screen unchecked.Are you saying that you go to Settings>Security>Device Administrators, try to check Android Device Manager, but aren't allowed to do so?
Yes,Are any other Device Administrators listed and checked?

Just for the heck of it, uncheck them, and see if the problem persists.
Hmm, see what happens if you go to Settings>Security and tap Clear Credentials.
