someone else may suggest a better ay but this works
copy
/includes/templates/template_default/sideboxes/tpl_categories.php
to
/includes/templates/YOUR_TEMPLATE/sideboxes/tpl_categories.php
then find:
if (SHOW_CATEGORIES_SEPARATOR_LINK == '1') {
$content .= '<hr id="catBoxDivider" />' . "\n";
}
right after that add:
$content .= '<a href="' . zen_href_link(FILENAME_LOGIN) . '">' . CATEGORIES_BOX_MYLINK . '</a>' . '<br />' . "\n";
changing FILENAME_LOGIN to where ever you want the url togo
then create a file here, called:
/includes/languages/english/extra_definitions/YOUR_TEMPLATE/categories_sidebox_extra_defines.php
put your defines in here like so:
<?php
define ('CATEGORIES_BOX_MYLINK','Login / Register');
?>
In the above example I added the login page below the categories <hr /> or line that seperates the categories from "New Products"
Hope this helps!