Note: If you can't see the Product Advertising API in your account, you should make 3 qualified sales and wait for amazon's approval to access the PA API feature.
Step 1: Log in to your Amazon affiliate account > Tools > Product Advertising API
Step 2: Click on Add Credentials button
Step 3: Click on Download Credentials button and save the Access Key and Secret Key
Important: You can't see your Secret Key again. So keep the downloaded file safe. In case if you lose it, you can create a new one but there’s a maximum limit of 2 active pairs of credentials per account.