Einstein was not directly involved in the Manhattan Project (which developed the atomic bomb). In 1905, as part of his Special Theory of Relativity, he made the intriguing point that a relatively large amount of energy was contained in and could be released from a relatively small amount of matter. This became best known by the equation E=mc2. The atomic bomb was not based upon this theory but clearly illustrated it.
In 1939 Einstein signed a letter to President Roosevelt that was drafted by the scientist Leo Szilard. Received by FDR in October of that year, the letter from Einstein called for and sparked the beginning of U.S. government support for a program to build an atomic bomb, lest the Nazis build one first.
Einstein did not speak publicly on the atomic bombing of Japan until a year afterward. A short article on the front page of the New York Times contained his view:
"Prof. Albert Einstein... said that he was sure that President Roosevelt would have forbidden the atomic bombing of Hiroshima had he been alive and that it was probably carried out to end the Pacific war before Russia could participate."
Einstein Deplores Use of Atom Bomb, New York Times, 8/19/46, pg. 1.
Regarding the 1939 letter to Roosevelt, his biographer, Ronald Clark, has noted:
"As far as his own life was concerned, one thing seemed quite clear. 'I made one great mistake in my life,' he said to Linus Pauling, who spent an hour with him on the morning of November 11, 1954, '...when I signed the letter to President Roosevelt recommending that atom bombs be made; but there was some justification - the danger that the Germans would make them.'".
Ronald Clark, Einstein: The Life and Times, pg. 620.
If he had been asked, I am sure Einstein would have sided with people like his friend Szilard who thought that the bomb should have first been "demonstrated" in a non-violent way for the Japanese, e.g. to have it go off in the middle of Tokyo Bay or at some other uninhabited area that was within view of a lot of people. But he was never asked. He never actually advocated the building of the bomb; he recommended that the nuclear weapons applications of fission be looked at by the government for purely deterrent reasons, and in only a very modest way.
Well he also was one of the eleven orignal signers of the 'Russell-Einstein manifesto', which was quite a warning against nuclear weapons and their use.
You can read it here: http://scarc.library.oregonstate.edu/coll/pauling/peace/papers/peace6.007.5.html
Einstein was a pacifist.