Before using, you need to register new application.
To do this, please follow URL: https://payments.amazon.co.uk/preregistration/lpa
Answer 3 questions and signup to start using the app.
Main page – you may now register the application (click button marked red on screen).
Do not forget to add URLs to be whitelisted.
You will need to copy your Client ID (it needs to be inserted into the offamazonpay.js and offamazon.redir.js in the proper place).
You would also need your Client secret key:
To check if you have proper keys, may test it online at Amazon MWS Scratchpad