'Login with Facebook' button not working on comments plus

Hi,

I am using comments plus on a multisite on wordpress 3.4.1. All the other buttons produce a popup except Facebook which does nothing. The plugin can be found here:

http://ashfordguitarlessons.com/guitar-gradesexams/how-to-achieve-top-marks-in-your-rgt-electric-guitar-grade-part-1/

Many Thanks
Paul Andrews