You can reach the Platform selection screen in your Publishwide Sync app by going to Settings > Platforms
Choose the sales channels that you want the app to sync:
Screenshot shows Amazon KDP selected and Barnes & Noble not.
Then choose the Ad Channels to sync:
Note: Amazon Ads has multiple marketplaces. You'll need to select each of the ones you run ads on. Below shows US, UK, CA & AU selected.
The app will then start logging into each platform. You may be asked to enter in OTP codes.
Then go back to the Publishwide Sync home screen and press "Start Sync"!
NOTE: Use different logins for your KDP and AMS accounts? You'll need to connect the two in order for the Publishwide Sync App to work correctly. Learn how by following these instructions.