Just relised you've posted in the WRONG section of the forum - this isn't for PureMart support!! No wonder I couldn't find any tabs..
anyway - transfer the class selected from templates/your_template/html/com_virtuemart/productdetails/default.php ~ line 668
<ul id="FlexibleProductDetails" class="shadetabs">
<?php if (!empty($this->product->product_s_desc)) { ?>
<li class="selected"><a href="javascript:void(0)" rel="#1product_s_desc"><?php echo JTEXT::_('COM_VIRTUEMART_PRODUCT_DESC_TITLE');?></a></li>
<?php } ?>
<?php if (!empty($this->product->customfieldsSorted['ontop'])) { ?>
<li><a href="javascript:void(0)" rel="#SpecificationsBottom"><span><?php echo $params->get('productdetails_specs','Specifications'); ?></span></a></li>
<?php } ?>
to
<ul id="FlexibleProductDetails" class="shadetabs">
<?php if (!empty($this->product->product_s_desc)) { ?>
<li><a href="javascript:void(0)" rel="#1product_s_desc"><?php echo JTEXT::_('COM_VIRTUEMART_PRODUCT_DESC_TITLE');?></a></li>
<?php } ?>
<?php if (!empty($this->product->customfieldsSorted['ontop'])) { ?>
<li class="selected"><a href="javascript:void(0)" rel="#SpecificationsBottom"><span><?php echo $params->get('productdetails_specs','Specifications'); ?></span></a></li>
<?php } ?>
in the same file around line 697 remove the display:none
<div id="SpecificationsBottom" class="FlexibleProductTABDetails" style="display:none;">
<div id="SpecificationsBottom" class="FlexibleProductTABDetails" >