Follow the below mentioned step to generate MWS keys from your Amazon Seller Central account.
Step 1: Login to your Amazon Seller Central and go to menu Settings -> User Settings as shown in the below screenshot.
Step 2: Copy the Seller ID from this screen as show in the below screenshot. This is your Merchant ID to be entered in the APP. Now click on the View your credentials link as shown in the below screenshot.
If you do not see the View your credentials link as above setup, click on the button Register as a Developer at the same page as shown below.
It may ask you for email verification. Enter the same email address which you are using for your amazon seller account and click on Verify Email.
Now you will be asked to enter your business/personal details same as which you have on your amazon account.
Step 3: A popup will gets opened as shown in the below screenshot. You can copy your Access Key ID and Secret Key. You may need to click on link View to get the Secret Key.