willie bee:
Did you check if this step was done right?

Using your favorite Text Editor, add the following code to the bottom of includes\templates\YOUR_TEMPLATE\templates\tpl_shopping_cart_default.php immediately above the last </div> tag.

<!-- bof Ultimate Cross Sell Combo -->
<?php require(DIR_WS_MODULES . zen_get_module_directory(FILENAME_CART_UPSELL)); ?>
<?php require($template->get_template_dir('tpl_modules_also_purchased.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_also_purchased.php'); ?>
<!-- eof Ultimate Cross Sell Combo -->

You could test if the problem is with this file by just deleting it from your template folder and see it the error goes away. If yes, I suspect you have a mistake in the code above.