On what page does Winston say that "freedom is the ability to say that two plus two equals four?"

In 1984, this quote appears at the very end of Part One, Chapter Seven, on page 84. To put this quote into context, Winston writes in his diary that the proles could overthrow the party, if they wanted to. The only obstacle to rebellion is the proles themselves: "Until they become conscious they will never rebel, and until after they have rebelled they cannot become conscious."

In this chapter, Winston also remembers a time when three party members were arrested for treason. Winston knew that they were innocent, having seen a photograph of them at a function when they were supposedly committing their crimes. But they were arrested and confessed publicly to all manner of horrible crimes. The men were eventually released and allegedly reinstated to senior party positions but Winston saw them in a cafe in the prole district about a year later. 

These two instances are very important in understanding the above quote. They demonstrate how the party is able to control people's concept of truth and manipulate their understanding of reality. The proles, for instance, are socially ousted by the party and portrayed as inferior. Over time, they have come to accept this status as truth and so they never pose a real threat to Big Brother.  Similarly, Winston destroys the photograph which proves the innocence of the three men. Though he does this through fear, it is illustrative of his job, more generally. He rewrites history, on the party's behalf, and erases  all historical events which the party does not want people to remember. The point is, then, that the party are capable of manipulating the truth to suit their agenda. If they say, for example, that 2 + 2 = 5, who will stand up against them and say that it is wrong? People are too afraid of the Thought Police and Room 101 to question what the party tells them and so, over time, they will come to accept whatever the party says as truth. As Winston says: "The Party told you to reject the evidence of your eyes and ears. It was their final, most essential command." In other words, the party are the creators of truth. 

In this understanding, then, it is only by avoiding the party's indoctrination that a person can truly be free. Winston uses the example of '2 + 2 = 4' to illustrate this point but, arguably, any idea that we conceive as 'truth' can be used. It is not the example which matters but the ability to develop one's own thoughts and to express them openly, without fear of violence. That is what Winston truly seeks and why he has become a rebel against Big Brother.

Please note that my copy of 1984 is the Penguin Classics, published in London in 1990, and the page number may differ slightly to yours.