look for the file: application/modules/User/widgets/list-online/Controller.php
then look the line:
if( $this->_getParam('showGuests', false) ) {
change it to
if( $this->_getParam('showGuests', true) ) {
then it works

Already shared here:
http://www.socialengineforum.com/index.php?/topic/9050-widget-to-display-online-guests-and-members/page__p__54899__hl__guest__fromsearch__1&#entry54899