Your first use of the Grand Slam Bomb is in the mission "Berlin Flak Tower". It's a red mission, very high risk.
It's part of the story, after the 6th critical mission. In the 7th segment of the story, 2nd from the end.(The spiky red missions on the map)
You are supposed to drop it from high altitude at level flight, which late in the game is easiest to avoid ground defenses and Flak. Just make sure your crew has warm gear to withstand the cold for long period.