Hi,
You've added your
<li id="hikashop_show_tabular_custom_li" class="hikashop_tabs_li ui-corner-top"><?php echo JText::_('Details');?></li>
inside the "if" condition to display the tabs of the vote/comment mechanism. But you didn't activate the vote/comment mechanism in your HikaShop configuration, and that's why you still don't see the tab.
Instead, add it after the code:
<li id="hikashop_show_tabular_description_li" class="hikashop_tabs_li ui-corner-top"><?php echo JText::_('PRODUCT_DESCRIPTION');?></li>
which displays the tab for the description.