Bluetooth headphones automatic connect
They should automatically connect as soon as you have paired them. They won't automatically reconnect if you take them out of range without turning them off first, to reconnect them, go into range and briefly press the call button on them. If you turn them off, then turn them on again and they don't connect, this is either a product issue, or your MacBook might not have the right settings.
Make sure that your MacBook is discoverable via bluetooth. If none of that works, contact LG and tell them about your problem, including all the steps you took to try and fix the problem. They will know more about it than me.