"Extensions" should appear in the tool, but this function is simply not there. I would like to add the Dash to Dock but https://extensions.gnome.org/ does not find the shell installing even though the chrome and Firefox extensions are installed.
From the screenshot it seems you have logged in to a Unity session, not a GNOME session. GNOME extension like 'Dash to Dock' is not supposed to work in Unity.
There should be a cogwheel (⚙️) next to the "sign in" button at the GDM login screen. Click on it and select GNOME instead of Unity (provided you actually have GNOME installed).