@aleph. The plugin is triggered in the Joomla Event, onBeforeCompileHead. However, this event was added in Joomla 1.5.23. You can check the change log to verify this: http://www.joomla.org/announcements/release-news/5367-joomla-1523-released.html
So, the event is not recognized in your version. To get the plugin to work, you will need to upgrade your Joomla version to the latest version 1.5.26. This will automatically make the plugin work.