Treble and Bass Cleff at the same time?

If you already have a score with a treble clef part for the melody, then from the menu choose Edit > Instruments. And from the "Instruments" screen, you can then select either Vocals > Bass or Strings - Plucked > Bass Guitar to add a bass clef line by clicking the Add button.

If you selected Treble clef as the template when you created your score and want to add a bass clef to it to make a piano grand staff, then Press I to open the Instruments dialog, on the right side of the screen click "Staff 1" under the piano and the click "Add staff" near the bottom in the middle (don't click "Add linked staff" you won't like the results). Press OK and you will have a grand staff.

But it sounds like maybe you are starting with the default empty score then trying to turn it into something else in terms of the arrangement of staves and instruments? While this is indeed possible, better to simply start a new that is already configured the way you want. Which is to say, ignore the default score, but go to File / New and create a new one, then select the most appropriate template there (eg, "Grand Staff").