This is a guide how to get this tool and how to activate it.
Firstly, download new Google Services and install it. (You probably have it already, so just replace it with the old one).
After downloading, you need to activate this app. To do so, navigate to settings -> security -> device administrators and allow Android Device Manager to make it work.
Was it really so hard?
Now, with working and enabled Google Services you are free to start managing your devices by visiting THIS site.