Dialogues are simply the places in a story where two or more characters are talking to each other. Find places in the book where Mrs. Reed is speaking to someone (Jane or someone else), and you'll have found a dialogue.
I find the first dialogue on page 1 of chapter I.
"What does Bessie say I have done?" I asked.
"Jane, I don't like cavilers or questioners: besides, there is something truly forbidding in a child taking up her elders in that manner. Be seated somewhere; and until you can speak pleasantly, remain silent."
[The first speaker is Jane. The second speaker is Mrs. Reed.]