All output text is defined in language files; there will not be "Add" anywhere in the files you mention. Line 135 in product_listing.php may be the place you are looking for:
PHP Code:
$lc_button = TEXT_PRODUCT_LISTING_MULTIPLE_ADD_TO_CART . "<input type=\"text\" name=\"products_id[" . $listing->fields['products_id'] . "]\" value=\"0\" size=\"4\" />";
You can edit the define for TEXT_PRODUCT_LISTING_MULTIPLE_ADD_TO_CART as the best way to simply remove the Add.
If you want to further edit the output, you can do it in product_listing.php.