Ican't figure out how to put in a voice
Do you mean a vocal voice singing the lyric or something like a soprano and alto on the same staff?
For the latter I suggest that you do the two voices on two different staffs and then enable layering. You can set rests to invisible and anything else that gets in the way. You need to be careful of the directions on the tail on each staff.
Enabling layering needs to be done in two different areas. In the page setup enable layering in the "contents" tab and then in staff properties tick "layer with next staff" on the "visual" tab for the top staff.
If you mean how do you (for example) record yourself singing and put that in a NoteWorthy file, you can't. Nor can you add actual human singing into any strictly MIDI music file. There are software sequencers that will allow you to coordinate both MIDI (instrumental) and audio (actual recorded sound such as a human voice) in the same composition. Cakewalk, Sonar, PowerTracks are some for $, one freeware package I know of (an internet search will reveal more) is AnvilStudio. Good luck!
For adding 'voice' and MUCH more. I recommend n-track http://www.fasoft.com I have been using it for > 1 year. It is easy to use. n-track can import Midi files (output from noteworthy) and convert them to '.wav' files. Noteworthy and n-track are a simply and affordable combination.