Google Checkout button not shown & gc_XmlBuilder class not found error

Hi,

I tried to install google-checkout-zencart-v1.4.7. I followed the instructions and copied/merged all the files. Also installed and set up on Admin. Now when I'm at the shopping cart page, the Google Checkout button is not there.

I used the debug logging util and found the following in the log:
PHP Fatal error: Class 'gc_XmlBuilder' not found in /home/myname/public_html/store/googlecheckout/library/googlecart.php on line 402

The line above is $xml_data = new gc_XmlBuilder();

So I checked and saw that gc_XmlBuilder was properly defined in googlecheckout/library/xml-processing/gc_xmlbuilder.php

Has anyone seen this error or any idea what might have caused this error?

Thanks for any advice in advance!