It appears that he was informed before the bombings, but only a few days before. However, this was probably more a matter of information security dotrine than of any lack of trust in the General.
Access to US intelligence information is based on two principles: Clearance level, and Need to Know. In order to have access to a military secret, a person must have both a clearance level at least as high as the level of the secret, and some need to know that secret. Clearly a General is going to have top-level clearance, but he or she might not always have a need to know.
At the time of the Manhattan Project, McArthur was the Commander of US Army Forces in the Far East, and then later at the very end (after the reconquest of the Philippines) he became Commander in Chief of all Army and Air Force units in the Pacific. However, there was one notable exception: the 20th Air Force. Perhaps this is because the 20th was coordinating with the Navy, not the Army. However, this is notable because the 509th Composite, the unit tasked with the "delivery" of the Atomic bombs was organized under the 20th.
We know the Manhattan Project was indeed being kept according to some variant of these principles, as FDR's own VP was unaware of it until after FDR died and he ascended to the Presidency.
The project itself seems to have fallen under the aegis of the Army Corp of Engineers, which were not under McArthur's command. It appears the chief of the US Air Force was breifed on the project by March 1944, as that's the date they had a meeting to discuss "delivering" the bombs.
In the absence of any specific information to the contrary, it would probably be reasonable to assume proper information security was being followed. In that case, General Macarthur would not have needed to know about the bombs before he became commander of AFPAC in April of 1945. One could argue that he still did not need to know, as knowledge of the bombings was clearly not to influnce the planning for Downfall, which was his primary responsibility, and none of the military units involved in the bombing were under his command.
Now as for specific information on MacArthur's exact date of breifing, I did dig up one article from a historian claiming that he was briefed only days before.
When first informed about their imminent use only days
before Hiroshima, MacArthur responded with a lecture on the future of
atomic warfare and even after Hiroshima strongly recommended that the
invasion go forward. Nimitz, from whose jurisdiction the atomic
strikes would be launched, was notified in early 1945.
("Early 1945" being February, just prior to the reorganization I mentioned above)