I believe that you are referring to something that happened in Chapter 3. Early on in the book, the family had been living down where they had first landed. But the mother, especially, wanted to live somewhere else that would be cooler.
In Chapter, 3, the father and some of the boys were out exploring and found a tree that they thought would make a good place to have a tree house. Later on in the chapter, they plan to move to that place. They refer to their move as a journey to the Promised Land.