popover doesn't open pop over


I've installed the plugin pop over on Wordpress Version 3.3.1 but it doesn't work.

Only the following plugins are installed and active:
- E-Newsletter
- WordPress Importer
- WPMU DEV Update Notifications
- Popover plugin
- Test Head Footer (this plugin did install together with "Popover plugin". When I deactivate, then Popover plugin gets deleted.)

Template installed:
WPMU Nelo 2.2.5


I receive the following warning in the backend.
Your active theme: Has the call to <?php wp_head(); ?> but it is not called directly before </head>

Cheers, Oliver