Once you have clicked to open a Google Checkout Account, you will find the Merchant Account as seen in the diagram.

First create an account... then note the graphic below to find the Merchant ID

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Your Google Merchant ID can be seen:

1)   At the top right of the Merchant Center pages (all).

If you are in Merchant Center, the top right section of the page will have your email address, a few navigational links, and right below that section is where you'll find your Merchant ID

2) It's also available in the INTEGRATION section of Merchant Center

   - If you're in Merchant Center, you'll see 4 navigational tabs namely Orders, Payouts, Tools and Settings (If you don't see these tabs, you are not in Merchant Center)

   - Click SETTINGS

   - On the left menu, click INTEGRATION. Your Merchant ID is also in the Account section of that page

Use this link to login to create a Google Checkout Account:

And this is where you sign up for an account.

Once you signed up, you will have your merchant id.  See the diagram above.

This is the link to an overview of signing up for Google Checkout.