Check the Engine.log file to see if there is a reported error.
They keyboard needs to have a kTISPropertyInputSourceCategory or kTISCategoryKeyboardInputSource and a kTISPropertyInputSourceType of kTISTypeKeyboardLayout or kTISTypeKeyboardInputMode to be recognised by Keyboard Maestro as a keyboard.
It only checks the former when setting the keyboard layout, so its possible you could use the Set Keyboard Layout action, export it as XML, change the XML to the keyman.inputmethod.Keyman ID, reimport the action and it might work.
The developers might be able to adjust the category or type, or if they are using a different category or type for some reason, I may be able to support another type.