Am getting the following error message when I trial a purchase on my website using google checkout (the same transaction works when using Paypal)

<?xml version="1.0" encoding="UTF-8"?>
<error xmlns="" serial-number="c4c75896-2d50-49d4-869c-f9bcc13030fa">
Bad username and/or password for API Access.</error-message>

I have triple check the Google Merchant ID and Google Merchant Key and they are all OK. I ahev posted this question on Serif help and Google and Mal's cart but no luck.

thanks in advance