Tone can be a difficult concept to understand, and some of the attached sites and the document may help, particularly the vocabulary list in the document which is attached.
As you look at the passage, what kinds of emotions or feelings are evoked by the way the author has crafted the sentences and paragraphs? Are there particular words that are tied to specific emotions, are there colors or descriptions in the passages that might also be there to build a certain feeling or craft a certain sense of emotion that goes along with the plot or exposition that is happening?
And you can look carefully not only at the words or the sentences but also in the way the story is developing, is it languidly moving along? Is it rushing to some kind of climax or new development? All of these types of questions should help you to get at and start describing the tone of a passage.