Thread:Extension talk:ArticleFeedbackv5/Class 'ArticleFeedbackv5Activity' not found

I just download this extension and I tried to install it on my MediaWiki 1.21 but I am getting this error when I go to mw-config:

Fatal error: Class 'ArticleFeedbackv5Activity' not found in mylocahost\wiki\extensions\ArticleFeedbackv5\ArticleFeedbackv5.php on line 461