Fill in a value for Command with Activator Device. This creates a device with that name. In your case it will create a dimmer device that you can/should use to control all your lights at once.
Under Activate Lights Options you need to
set up indicator usage for groups (any or all)
enable 'Activate as Group with Zigbee Group Commands'.
'Zigbee group commands' is what eliminates the popcorning as it sends a single signal to all the lights.
