Where to find your Merchant ID number in Google Checkout:
1) Go to google.com/checkout and sign in.
2) Click on "My Sales".
3) Highlight and copy your Merchant ID number.
4) Go the administration portion of your website and:
a. Click on the Settings module.
b. Click on the Advanced tab.
5) Scroll down and paste your Merchant ID number in the available field. Scroll down further to click on"Save settings".
You have now combined your Google Merchant account with your website.