Try replacing
<?php include(DIR_WS_BOXES . 'categories.php'); ?>
with
<?php include(DIR_WS_BOXES . 'dm_categories.php'); ?>
in column_left.php
and add
<?php echo $GLOBALS['dmfooter']; ?>
just before the </body> tag in the footer.php
Check inside google checkout and make sure its not set to https response if you are using response.php on http. You need a secure certificate installed to use https
If your hosting allows it outside the root folder would be better. If it is not allowed put it in some folder deep inside your OSCommerce install and set the permissions to not readable from the web.
I agree. Most professionals don't know everything off head either. Please try searching for the info on google or other search engine . If it still doesn't work for you . Try getting someone to do it for you like a freelancer. ;)
We have actually solved that problem before. The problem might be with 2Checkout. Lat I remember they will pause your account when they see that there is a problem or they are missing information. When they do it this error will occur.