Which is the Num key on the keyboard?

There is no Num button. The shortcuts mean the + or - that is on the number pad (image source):

If this image of an Z560 keyboard that I found online is correct, the buttons you are looking for are circled in red (image source):

In your case, it likely refers to the + and - keys of the numeric keypad:

Num+ / Num- probably means the Plus or Minus Key on the Numpad. If you don't have a separate Numpad on your Notebook, you have to use another Key (in most cases it's labeled Fn) in conjuntion with that QWERTY-Key which is labeled + or - in the upper right corner.