Module is not supported anymore.
Check out Google Checkout code from Google http://code.google.com/p/google-checkout-oscommerce/

Sign up for a seller account	Buy and install Google Checkout for your cart

$50 AdWords Coupon

For a limited time, when you sign up for Google Checkout through the below link, you will receive a $50 AdWords coupon.

Notice: This special offer is valid only for new Google Checkout merchants with a self-managed AdWords account. Merchants must (i) be properly referred by Qualiteam Software, (ii) use an approved implementation of Google Checkout, and (iii) successfully process at least three (3) valid payment transactions and at least $500 of valid payments as a merchant through Google Checkout. The promotional credit will be distributed directly to the merchant within 30 days after the end of the month in which the merchant satisfies the criteria outlined in (i) and (ii). Merchants will be charged for advertising that exceeds the promotional credit. Merchants will need to suspend their ads if they do not wish to receive additional charges beyond the free credit amount. Subject to ad approval, valid registration and acceptance of the Google AdWords Program standard terms and conditions. The promotional credit is non-transferable and may not be sold or bartered. Offer may be revoked at any time for any reason by Google Inc. One promotional credit per merchant. For new AdWords Program registrants, an activation fee of $5 will be applied and deducted from the promotion credit. Offer valid only for merchants in the United States signing up for Google Checkout through the referral link provided. The promotional credit will expire approximately 90 days from the date the credit is issued.

osCommerce Google Checkout payment module is designed to be used with osCommerce and Google Checkout Payment system. After installing you receive Google Checkout button under your store checkout button, as recommended by Google.

We HIGHLY RECOMMEND you to add this payment method to your store.
Customers love Google - they will love your store!

This package contains all files needed plus an updated readme file with installation notes. This module uses Google Checkout API, Level 1.

How module is working

Module is passing shipping costs to Google Checkout, product attributes price is included in products price. Taxes are also passed to Google Checkout (i.e. supported). Current functionality work for 99% of osCommerce stores.

Latest installation instruction located here
http://docs.magneticone.com/?product=oscommerce_googlecheckout

We are working now on Google Checkout Level 2 integration.

Technical requirements: Zend Optimizer 2.0+ or ionCube Loader (files are encrypted) Important: View additional technical requirements Instant product delivery You will receive product in 1-2 minutes