##Andy, I still have problems with that

Here's how I load the plugin now.  I put the plugin into a new class; and the Extension class is stored in another file which is called at the init.

////////
////////
$classified3s_groups = new Classified3s_Groups();

if ( defined( 'BP_VERSION' ) )
	$classified3s_groups->init();
else
	add_action( 'bp_init', array(&$classified3s_groups,'init'));


##Classified3s_Groups

class Classified3s_Groups {
	function init(){

		define ( 'BP_CLASSIFIED3S_PLUGIN_NAME', 'buddypress-classified3s' );
		define ( 'BP_CLASSIFIED3S_PLUGIN_DIR', WP_PLUGIN_DIR . '/' . BP_CLASSIFIED3S_PLUGIN_NAME );
		define ( 'BP_CLASSIFIED3S_PLUGIN_URL', WP_PLUGIN_URL . '/' . BP_CLASSIFIED3S_PLUGIN_NAME );
		require_once ( BP_CLASSIFIED3S_PLUGIN_DIR . '/bp-classified3s-groups-classes.php' );
		bp_register_classified3_extension( 'Classified3_Groups_Extension' );
...

##still have the error :
Fatal error: Class 'BP_Classified3_Extension' not found in F:\web\kinetest\wordpress-mu\wp-content\plugins\buddypress-classified3s\bp-classified3s-groups-classes.php  on line 3