Amazon requires that you acquire your AWS key by following the steps below. You can not just use an AWS key you use for S3 storage etc.
The Amazon key setup for new Amazon Associates has three parts: Associate Registration, API Registration and AWS key generation. It is important to do the steps in order.
1) Associate signup:
Select the Join now free button
2) Api signup & AWS keys:
After signing up as an Associate you need to get api access.
You can get these from the tools menu in your associate account.