Atomic Command is based on Missile Command. The aim is to shoot down falling missiles before they destroy the world.
Follow the southeast road from Sanctuary to reach the Town of Concord, where you'll find the Museum of Freedom landmark.
Enter the museum and prepare to fight your way through a few Raiders.
When you reach the top floor, enter the room at the end of the corridor where the survivors were pinned down.
The magazine with the Atomic Command holotape attached is on the desk next to the terminal. It's missing from the screenshot above, but rest assured, that's where you'll find it.