The Merchant ID is found by visiting your storefront and looking at the URL. The alphanumeric value directly after "me=" is your merchant ID. Here is a random example: A3NR5VAI6LJPGE. To get to your storefront, go to your Seller Central account, hover over "Settings" then click "Account Info." At the top left, you should now see a link for "Your Seller Profile," click there. Midway down on this next page you should see a link to your storefront.