Because the climax of a story is the point of greatest drama, the only way to find the climax is to read through and notice when everything comes together. In The Twisted Window, this is very close to the end: the moment when Jamie and Tracy confront Brad at the cabin and the gun goes off. It's so important we get the scene twice, and it leads to the darkest secret of the book.
As for the conclusion, that's a bit simpler. That's how things end up. Jamie reassures Brad and Tracy lets Cricket know they'll be taking her home.