If you are a seller on Amazon's UK marketplace then finding your MWS credentials is a little different than the US-based .com site.
1. Log in to your Amazon Pro Seller Central account -> Click “Appstore” -> Then click “Manage Your Apps“:
2. Select ‘Authorize new developer‘:
3. Add AccelerList ‘Developer Name‘ and ‘Developer ID‘:
Developer Name: AccelerList
Developer ID: 11364433437
4. Copy and paste your Seller ID and MWS Auth Token into the AccelerList onboarding section and you should be able to proceed.