i just solved this problem today (i'm assuming it's the same problem you're having).
the answer is in the "tracking users and orders" section of the "Readme" document that came with the google checkout files you (should have) downloaded.
excerpt from "readme"
INSTALLATION NOTES
==================
1. Follow instructions contained in the INSTALLATION file.
2. Verify the installation from the Admin site and selecting MODULES->PAYMENTS
and checking if Google Checkout is listed as a payment option.
3. Set the file attribute to 777 for /googlecheckout/logs/response_error.log and
/googlecheckout/logs/response_message.log files.
4. Go to http://<url-site-url>/googlecheckout/responsehandler.php
,
or a request for User and Password, go to the next section.
If you get any errors, you must correct all errors before proceeding.
Refer to the troubleshooting section below or go to the support forum for help.
#4 is very important... replace "<url-site-url>" with the path to your /googlecheckout... folder on the host server.
eg.
http://www.zippers.com/zencart/googl...nsehandler.php
If you get a 'Invalid or not supported Message', 'Fail to get HTTP Authentication', you're on the right track.
you must also have a google analytics account set up.
excerpt from readme:
Enabling E-Commerce Reporting for Google Analytics
==================================================
To track Google Checkout orders, you must enable e-commerce reporting for your
website in your Google Analytics account. The following steps explain how you
enable e-commerce reporting for your website:
1. Log in to your Google Analytics account.
2. Click the Edit link next to the profile you want to enable. This link
appears in the Settings column.
3. On the Profile Settings page, click the Edit link in the Main Website
Profile Information box.
4. Change the selected E-Commerce Website radio button from No to Yes.
More info:
http://code.google.com/apis/checkout...tegration.html
next, log into your google checkout account and click on the "my sales" link at the top of the page and click on the "settings tab". when you get to the settings tab, navigate to "integration" using the links on the left side of the screen.
now you should find a section labeled "API callback URL (Level 2 integrations only)"... in the "API callback URL: box, you must type the url to the responsehandler.php file.
eg.
http://www.zippers.com/zencart/googl...nsehandler.php
that should do it!!
btw
here's the excerpt from the readme dealing with tracking:
TRACKING USERS AND ORDERS
=========================
In order to provide this support (as required for Level 2 integration), update
the API callback request field in the seller account to
https://<url-site-url>/googlecheckout/responsehandler.php .
Note that the production Checkout server requires an SSL callback site.
The Sandbox server accepts non SSL callback URLs.
View your Google Checkout customers and their orders in the Reports tab.
For each order, the default starting state is Google New.
GC Orders States in ZC Admin
GOOD LUCK
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