First, you will be asked to Sign Up for MWS
Next, you will log in using your normal Amazon credentials
Then you will need to select the top option, I want to access my own Amazon Seller Account with MWS
You can review the terms and conditions
Finally you will be presented with the keys. You will need to copy and paste these back into the PSS screen.
Once you have pasted the correct keys in PSS please hit Save at the bottom of the page.