how add bass clef

In the same dialog you can also add a staff to an existing Instrument, second to last button

That's assuming this is intended to be for piano, so the two staves should be connected. Or just add "Piano" as an instrument - which automatically adds connected treble and bass staves - and delete the existing staff. If you are adding the second staff for a second instrument, you add the instrument, not just a staff. But in any case, Edit / Instruments is *always* the place to go.