Hmmm... You did it differently than I did. I added a new "block" to sts_user_code.php:
$sts_block_name = 'cat_tabs';
require(STS_START_CAPTURE);
require(DIR_WS_BOXES . 'categories_tab.php');
require(STS_STOP_CAPTURE);
$template['cat_tabs'] = strip_unwanted_tags($sts_block['cat_tabs'], 'cat_tabs');
then called it in sts_template.html:
<!-- Begin Category Tabs Mod -->
<table width="100%" border="0" cellpadding="0" cellspacing="0" class="TabsSides">
?<tr class="TabsNavigation">
? ?<td align="center" valign="bottom" nowrap class="TabsSides">$cat_tabs</td>
?</tr>
</table>
<!-- End Category Tabs Mod -->
I didn't have to change any of the other files to get it to work (except for adding some style changes to make it look like I wanted).