Both an access key id and a private (secret) key are necessary in order to access the Amazon Web Services Product Advertising API. This is the process for obtaining one:-

1. You need to create an account at http://aws.amazon.com - this is free.
2. Once you have created your account, you sign into your console at the same address. Ignore the message to sign up to EC2, you don't need it.
3. Click on your account name at the top right of the page, then click on security credentials
4. You may get a pop-up message, if so, click on 'continue to security credentials'
5. Look down the page to 'Access Keys'. If you do not yet have an access key, you will need to click the button that says 'create new access key'. Otherwise, your access key id will be displayed in the table under 'access key id'. Your private (secret) key will not be displayed. When you create a new access key you will be given the secret key, which you must store securely, because you will not be able to recover it later. If you do lose it you will need to create a new access/secret key pair.

As its name suggests, you should keep the secret key to yourself. This is used to digitally sign the requests that you make to the Amazon Web Server.

When signing up for your access key ignore all the stuff about IAM management, it is not relevant.