If you want to get your Amazon Product Advertising API key and secret, you need to have an account at Amazon Associates. I'll assume that you already have one, otherwise you wouldn't be interested in Amazon Product Advertising API.
You need your Product Advertising API key to add it to apps or plugins, such as Auto Affiliate Links.
After you log-in into your account, go to: Manage Credentials page, or in main Amazon Associates menu, go to Tools, and then click on "Product Advertising API".
On that page, scroll down until you see "Manage Credentials" section. There, click on "Add Credentials". You will see your API key and secret, that you have to copy and store in a safe place.
This is a catch, you can’t get the API key before you make 3 sales…. and you can’t make sales until you put the API key into the blog…
Unfortunately, this is the way Amazon is working right now. Until you make 3 sales, you can generate links and add them to Auto Affiliate Links.