To use Google Checkout, first sign up for a Google Checkout account. Then just navigate to Payments under the System tab. Click on Edit in the Google Checkout column and enter your Merchant ID and Merchant Key (provided by Google upon sign up). Make sure to select Enabled from the Status drop down box.
Once this is set up, you will be able to start selling in Google! Some important things to remember about Google Checkout:
1) Your customers will not be required to login to your site, and the Google Checkout option will only show up in the shopping cart.
2) The entire checkout process takes place within Google, not your MiiDuu store.
3) Your Miiduu shipping methods are NOT linked to your Google Checkout account, so you will need to set up shipping in your Google Account directly.