In Store -> Payment -> Google Checkout, you could enable Google Checkout along with your credit card payment. Before you configure Google Checkout, make sure you have signed up Google Checkout account. To enable "Google Checkout", click "Enabled" checkbox. Enter other neccessary fields that are required to complete Google Checkout payment module. Here are some field explanations.

• Use Sandbox: when you enable this, all the transactions are redirected to Google Checkout Sandbox which is a testing environment. If you want to test Google Checkout model, you could enable this. Be sure to turn it off before go live.
• MerchantId and Merchant Key: login to your Google Checkout account. In Settings -> Integration, you will find your merchant ID and key on the right side of the page.

Do not forget to click "Save" button when you are done.

You are not done yet. You need to configure the API callback URL in your Google Checkout account. Go to Settings -> Integration in "API callback URL" text box enter https://[your store URL]/Store/GoogleCheckoutPostHandler.aspx.

*[your store URL] is the web url of your online web store. You can find it under Store -> General -> Store Address -> Store URL.