The Epic game Launcher application for mac os is broken in version 5.3.1.
To install the multiscreen plugin, you need to get the source code in the archive from the developer, then compile this plugin via xcode for the Engine 5 compiled from the github sources 5.3.1. Next, copy the received files (files are created there in the binary folder) to the UE_5.3/Engine/Plugins/Marketplace/MultiScreen folder.
After that, everything will work in the application, although the plugin will not be displayed in the list of connected in epic game launcher.
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