It show like this:
"Warning: Issues were found with your site. Please check all tests below and correct any warnings or errors.
Test: JFBCSystem plugin is enabled
Fail: Not Detected! Please enable in the Site Config area of Autotune or in the Joomla plugin manager.
If the JFBCSystem plugin is enabled, but you still get this error, please check:
Your site is not in Offline Mode. If so, turn your site Online and re-run the Error Checker.
If you have a 'mobile template', disable it and re-test
Some templates have their own Facebook integration, and actively try to remove other Facebook extensions files. Try a different template.
No further tests can be run until fixed.
If you are having issues, please ensure your Facebook Application does not have the "Sandbox Mode" setting enabled. You can check that setting in the Facebook Application Area in the "Basic" section."
The plugin is enable and the site is not in offline mode. I use Yootheme template in my sites and it always works.