The weird thing is that joomla! load by itself the file toolbar.kunena.php in Kunena frontend...
For me Kunena 1.7.2 works perfectly in J! 2.5.0 Beta1.
JToolBarHelper is loaded automatically in administration and in the site Kunena doesn't load the file.
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This is a Joomla bug reported in here: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=27567
Fix #214 Fatal Error: Class 'JToolBarHelper' not found