Try again, sometimes if you take too long too enter your the user and password your server might time out and give you the 404 error.
Try to access the googlecheckout folder again. If it's configure correctly, you should be able to use your merchant id as user and merchant key as password.
If it doesn't work, you probably did not set up the .htaccess and .htpassword files correctly.
Are you running php over cgi?
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