Login to your Amazon Seller Central and go to menu Appstore -> Manage Your Apps as shown in the below screenshot.
Now click on button Authorize new developer as shown in the below screenshot.
Input Developer's Name and Developer ID based on the Amazon marketplace you are selling on and click on the Next button as shown below screenshot.
For amazon.com, amazon.ca and amazon.co.mx:
Developer Name: infoshore
Developer Number: 7519-1546-3728
For amazon.co.uk, amazon.in, amazon.de, amazon.fr, amazon.it and amazon.es:
Developer Name: infoshore
Developer Number: 7336-8239-2608
Accept the terms and click on the button Next as shown below screenshot.
Now you will see your MWS keys as shown below screenshot. Copy Seller ID and MWS Auth Token from this page.