We should not apologize for the attack
We, as the Allied nations of WW2, should not apologize. First, it wasn't just the United States who built it, Canada supplied the uranium, and many other nations provided their best scientists to build it.
The reason why Pearl Harbour is so remembered by the American people is that Pearl Harbour was a suprise attack, while the bombings of Hiroshima and Nagasaki were not, as well as the Japanese emperor had the chance to surrender to the Allies after the first attack, but didn't, so they used the second one.
Also, when they massacred millions of surrendering Chinese troops and the brutality against the captured Americans, Australians, etc., the few hundred thousands that were killed by the bombs was nothing compared to the millions killed by them.
They will never get an apology from anyone.