After world war 1, president wilson went to europe to write the peace treaty that ended the war. was he within his constitutional powers when he negotiated a where is it addressed in the article and section) did the treaty activate as soon as he signed or why is it addressed?

YES, President Wilson was within his Constitutional powers when he negotiated terms of the Treaty of Versailles, which came at the end of World War I.   This power is assigned to the President in Article II, Section 2 of the Constitution, which states, "The President shall have Power, by and with the Advice and Consent of the Senate, to make Treaties."  The advice and consent of the Senate typically comes after the treaty has been negotiated by the President.  As reported by Justia (a legal information site), "The Senate’s role in relation to treaties is essentially legislative in character."  In a Supreme Court decision in 1936, this was affirmed by the Court's decision, which said, that the President "alone negotiates. Into the field of negotiation, the Senate cannot intrude" (United States v. Curtiss-Wright Export Corp., 1936).  

NO, the treaty President Wilson negotiated did not activate as US policy as soon as he signed it.  The Senate refused to ratify the treaty.  This is addressed also in Article II, Section 2 of the Constitution, which gives the Senate the authority to confirm treaties negotiated by the President.  The full phrase in that section of the Constitution says of the President, "He shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur."

Details about the Senate's refusal of the Treaty of Versailles:

The League of Nations was the signature idea of US President Woodrow Wilson.  He had laid out 14 Points for establishing and maintaining world peace following the Great War (World War I).  Point #14 was the establishment of an international peacekeeping association.

The Treaty of Versailles adopted the idea of forming such an international association, which became the League of Nations.  But back home in the United States, there was not support for involving America in any association that could diminish US sovereignty over its own affairs or involve the US again in wars beyond those pertinent to the United States' own national security. Because of its objections to membership in the League of Nations, the United States Senate refused to ratify the Treaty of Versailles.