In the 1997 Random House edition of Into the Wild, the quotation you mention is on page 191. Of course, as you can see here, by reading both of these responses, the different editions contain different page numbers.
Further, if you have an online edition or one for the kindle (for example) or other eReader, there may not be a page number at all! (That is because with an eReader you are able to search for certain words throughout the entire book.)
I thought it might help you even more to put the tiny quotation into the greater context and include an explanation:
“And so it turned out that only a life similar to the life of those around us, merging with it without a ripple, is genuine life, and that an unshared happiness is not happiness.... And this was most vexing of all," he noted, "HAPPINESS ONLY REAL WHEN SHARED.”
Happiness, then, can only be achieved when merging experiences of life with the experiences of others and having "a life similar" to others. In essence, this is a part of the text about sharing that happiness and is one of the most important quotes from the book.
In my copy of Into the Wild, the 1996 Anchor Books edition, the quote
"HAPPINESS ONLY REAL WHEN SHARED"
is on page 189. As page numbers vary from edition to edition in books however, it will be more useful if I tell you that the quote can be found on the second or third page of Chapter 18. Christopher McCandless wrote these words in the margins of the book Doctor Zhivago by Boris Pasternak. He scribbled it next the a passage which reads,
"And so it turned out that only a life similar to the life of those around us, merging with it without a ripple, is genuine life, and that an unshared happiness is not happiness...And this was most vexing of all."
Some interpret McCandless's notation as evidence that his sojourn in the wild had changed him, and that he was ready to return to civilization and "become a member of the human community." The world will never know for sure if that was true, however. Chris McCandless died three works after writing these words; Doctor Zhivago was the last book he would ever read (Chapter 18).