I just barely downloaded NWC 1.70 (so you'll probably see me on here alot!) :O) so how do I hide notes, rests, and other symbols?
Select the desired item or items (i.e. highlight it by click/dragging with the mouse or using Shift + L or R cursor on the keyboard) then either "Edit | Selection" with the mouse or CNTL-E (or ALT-Enter) from the keyboard, and click on the "Visibility" tab. Viola. :)
The help file explains each of the new features, and how to activate them. It is in the Common Questions section.