What could General MacArthur have done more to stop the using of the atomic bomb on Japan?How did he express what needed to be done to end the war...

In a word, nothing.  This topic has come up fairly often
in recent days, and the consensus seems to be the same: the decision to drop the atomic
bomb was made because of a desire for revenge and to shorten the war.  The decision was
made at the Presidential level, even before we had the bomb developed for testing.  So
MacArthur, while he was the Supreme Allied Commander in the Pacific and had a great
amount of respect and influence on military matters, would not, in my opinion, have been
able to stop the bomb from being used.  He may have been able to influence the
timing of its use, but that's about it.  He later admitted himself
that he had not even been consulted about the use of the
weapon.

That being said, MacArthur did have a military
opinion on it.  He believed it was militarily unnecessary.  As we did not accept an
unconditional surrender of Japan, but a conditional one where the Emperor would retain
his position but not his power, he felt that concession might have prompted the Japanese
to stop fighting without the use of the bomb.  Unfortunately, we'll never
know.