Userscripts.org

Have you read the warning in the description?
"Once installed, you have to edit the script to fill in your own Amazon Access Key ID and Secret Access Key"

Go to http://aws.amazon.com/, login or create yourself an account, then go on the "Security Credentials" page of your account. Search your your access keys and will find your "Access Key ID" and your "Secret Access Key" information.

Then edit the GreaseMonkey script (GreaseMonkey -> Manage User Scripts -> Amazon barcode -> Edit) and fill up the missing information:

//////////////////////////////////////////////////////////////////////////////////////
// /!\ These parameters need to be filled up by each user, otherwise this script won't work
//////////////////////////////////////////////////////////////////////////////////////

// User Amazon Access Key ID
var AWSAccessKeyId = '';

// User Amazon Secret Access Key
var AWSSecretAccessKey = '';