Google Checkout has mostly been renamed to Google Wallet.
However, setting up Google Checkout is still something that is independent from Google Wallet. Getting your credentials for Google Checkout is a three step process.
1) Sign up for Google Wallet. This is free, and you don’t need a credit card.
2) Sign up for a Google Merchant Center account and create a virtual store front for yourself: www.google.com/merchants
This allows you to link with the next step, which is actually your google checkout account.
3) Sign up for Google Checkout using the link provided. This link allows you to become a Google Merchant and adjust the settings to find your credentials.
Agree to the terms of service after filling out all the required information, and move on to making your actual merchant store. Proceed to the settings tab, and under integration is where you can find your credentials.
For any other questions about setting up Google information as a merchant, please read this article: http://support.google.com/checkout/sell/bin/answer.py?hl=en&answer=42963