CAS plugin not active

[pyro_fire]

I'm trying to use the Roundcube-CAS-Authn plugin to enable CAS authentication on a Roundcube installation I have here.
At first I copied the plugin folder into the Roundcube plugins folder, edited it and named the file to config.inc.php.
Next I added the 'cas_authn' entry to the $config['plugins'] variable.
However I still see the Roundcube login homepage, it seems that the plugin has not been activated.
Has anyone ever experienced this?