View more information about: Google Checkout | Merchant Key | Merchant ID.
To find your Merchant ID/Key you need to first go to http://checkout.google.co.uk/ and login.
You will then be taken to this page:
You must click on My Sales to go to the next page:
This will take you to the settings page:
Click on Integration to go to the next page:
Your Merchant ID and Key will be on the right side of this page.