Login to your SellerCentral Amazon Portal. At the main dashboard find the Integration central section.
On a new page, you be able to find the API access section. In this section, you will have almost everything you need. Also if you click the See details button, you will get all-in-one shown to prepare configuration, except "Private Key".
The last thing to configure payment is the private key. This is generated automatically when creating keys, as a .pem file. Need to download it to your hard drive, as this can not be stored at the seller central dashboard.
When adding a key to the configuration of the plugin, copy exactly this same way it was generated, including "--Begin" and "--End" part of a .pem file.