Click on the Staff Tool and then double click on a staff. Lower left corner - under Independent Elements choose Key Signature. Then when you choose the Key Signature Tool and change the key of a staff, only that staff will have a new key signature.

THANKS!  Exactly right!  IS it possible for me to have one staff in one time signature, like 4/4 while another is in 6/8?

Sure - same thing - just select the option for a staff to have an independent time signature. I do this all the time and there's one thing to be really careful about - make the time sig change before you enter any notes! Also - copying a measure to another staff can be tricky.