When upgrading to the Apple App Store version of Mattermost desktop on first start you may see the message
Due to some pathing changes in changing the desktop application to the App store you will need to import you previous configuration from the legacy storage location of the Mattermost Desktop app. This profile will include any settings you've saved such as server lists, personal preferences, etc. You can choose "Select Directory and Import" and navigate to and select the Mattermost directory in
This should import your configuration and allow you to use Mattermost.
If you do not want to carry over your previous settings you may select "Don't Import" and work with a fresh install of Mattermost Desktop