An Atmos is locked with the lever below the dial, it is locked in right hand rotation so that when the clock is set up and the lever is released by pushing to the left the clock will start on its own. It should remain locked until you are ready to set the Atmos clock in its permanent place. Moving the Atmos clock without locking the pendulum can cause serious damage to the movement.
The correct time to lock the pendulum is when it reaches an extreme swing, i.e. it stops moving. Do not lock the clock whilst the pendulum is in mid-rotation. The locking lever is quite stiff so push as gradual as you can whilst holding the clock steady.