new staff in score but not as part

the title says it all really. I am adding a new part to piece (a revoicing for a different instrument). I have copied the notes across ok, and they look and sound right. However I cannot see a new part tab to view the part as it will print. How do I add that?
I am very new to musescore, so I need a duumies explanation. I have used noteworthy in the past,

thanks.