How to use many clefs

Difficult method:
In the instruments dialog (press i to see it) you can select the bass clef and click add staff for another staff. To make it be above the bass clef, select the new staff and press the up arrow button above the "Add Staff" button to move it up. If you already have notes on your bass clef you will need to do this, if you have not added any notes yet, then don't worry about move up the staff. Click OK and you will notice that both clefs are bass clef, use the palette to put a treble clef at the beginning of the staff on top.

Easy method:
When you create a score add an instrument that is normally written on a grand staff (like a piano) and you will automatically get both clefs. If you need to change the instrument, then right click the staff, select staff properties then click the change instrument button near the middle of the box and select the instrument you want.