Unable to call next tab in tabbed form

Hello,

I have a tabbed form in Joomla 1.5. The tabs look great the form and all its js functions work brilliantly. My problem is that the buttons that are coded to send the user to the next tab while they worked in Joomla 1.0, do not work in V1.5.

<?php
jimport('joomla.html.pane');
$pane =& JPane::getInstance('tabs', array('startOffset'=>0));
echo $pane->startPane( 'test_form' );
echo $pane->startPanel( '<b>Class Info.</b>', 'tab_1' );
?>
HTML code
<p align="center"> <input type="button" value="Start Registration!" onClick="tabPanel.setSelectedIndex(1);"/> 

It keeps coming up as 'tabPanel' is undefined.
I would appreciate any assistance anyone could give.
Thanks,
Marty

Joomla! forums.