This is what is says in the read me file. But it is incorrect.

https://<url-site-url>/admin/googlecheckout/responsehandler.php
Chances are you do not have the googlecheckout folder under the admin folder, so it will most likely be:
https://<url-site-url>/store/googlecheckout/responsehandler.php
or
https://<url-site-url>/googlecheckout/responsehandler.php


You logon into google checkout. Click settings tab. Then you will see an Integration in the left. Then drop your url into the API callback URL text area.