Hi Frederic, the button shows up without problem in the default editor button array. It looks like you are doing something custom with the editor. Have a look at the main php page in the plugin, openbook.php. In particular, look at these lines:
add_filter( 'mce_buttons', array( $this, 'filter_mce_button' ) );
add_filter( 'mce_external_plugins', array( $this, 'filter_mce_plugin' ) );
add_filter('mce_css', 'filter_mce_css');
You will require the indicated functions as well: filter_mce_button, filter_mce_plugin, filter_mce_css. These are included further down the page. Your implementation may require a variation on this, but that's how I include them in the default editor array. Hope that helps.
John