I just embedded mine right in the cart labs template, instead of putting the content in it's own template and then calling that. Then I wrapped a conditional to show it if the product was in a specific category (since that's what I needed).
The Readme file is pretty easy to follow.
New Tab
Code:
{if $product.categoryid == 26}[*]
Guarantee{/if}
New Tab Content
Code:
<DIV id="guarantee" style="display:none;">
{capture name=dialog}
<table border="0" cellpadding="0" cellspacing="0" width="100%">
<tbody><tr>
<td class="DialogTitle" background="/xcart/skin1/images/dialog_bg_n.gif" height="15" valign="bottom">Event Satisfaction Guarantee</td>
</tr>
<tr><td class="DialogBorder"><table border="0" cellpadding="10" cellspacing="1" width="100%">
<tbody><tr><td class="DialogBox">
<h2>Money Back Guarantee</h2>
[img]images/guaranteed1.jpg[/img]Here's the deal.
...
</td></tr>
</tbody></table></td></tr>
</tbody></table>
{/capture}
{include file="cartlab-tabs-dialog.tpl" title="Money Back Guarantee" content=$smarty.capture.dialog extra="width=100%"}
</DIV>
Just don't forget to add the new tab to the tabs.js array (again, it's in the readme).
Here is how I implemented it:
http://www.empowermentgroup.com/xcart/product.php?productid=16134&cat=26
Hope that helps