If I try adding a new staff, it just adds a 3rd staff to the first line but I want to add a whole new line under the first one. Thanks.
How many lines you get is determined by the length of the piece, margins, and font size. To see the lines you have to look at print preview. The staves in the editor where you are working just continue as one line until you are done.
I interpret your question in different possible ways. Perhaps these links will help:
FAQ - Is there a line break command?
http://ntworthy.com/composer/faq/33.htm
FAQ - How do I enter more than one staff?
http://ntworthy.com/composer/faq/3.htm
Move Staff
http://ntworthy.com/nwc2/help/MNU_MOVESTAFF.htm
Something that may be causing you confusion is that when NWC adds a new staff, all it does is give you the system opening barline. You will have to add everything else - clef, time signature, key signature, and all the notes, rests, and barlines. To fill in the new staff with rests to the point where you need it, add the clef, time sig, and key sig, then set note length to whole note and press <spacebar><tab> repeatedly until you are under the point in the music where you want to begin adding notes. The spacebar gives you rests, which will be whole-measure rests with this setting; the tab gives you barlines.
If you are writing a second instrumental or vocal part that is in unison with the first staff for a number of bars, then goes divisi, after you add the new staff, just copy and paste the contents of the first staff onto the second up to where you want the divisi to start.