Question:  when I add a new instrument (for example "Kontakt") to an existing rack file and then open the Rack-Editor it is not possible to find the buttons for "Mute" in the scenes.

Answer:  This is actually by design.  Forte will only show controls for the scenes that have specific instructions for a plugin.  If you had previously created a scene and then later added a plugin, the scene does not have any information or configuration for that new plugin.  It doesn't "know" about it, so it has no configuration to show.  If you update the scene or run the "Trim Rack" command which rebuilds all the scenes, each scene will then know about the new plugin.