How can I fix or diagnose problems when Find My Mac doesn't work?
Updated

Here is the explanation from Apple's iCloud Troubleshooting:

I cannot turn Find My Mac on

This can occur if:


You are not an administrative user on your Mac. "Administrator authorisation required" will appear under the Find My Mac switch in the iCloud System Preferences pane. Log out of your account, log back in with an administrative user account and try to turn Find My Mac on in that account.
Your Mac has an old Recovery partition. "Recovery system update required" will appear under the Find My Mac switch in the iCloud System Preferences pane. From the Apple () menu, choose Software Update and make sure you install the Mac OS X Lion Recovery HD Update.