Once you have opened the case, there is a black on/off switch on the right side. When you switch this button to on, the computer will start up. When you wish to turn off the box, flick the switch to off.
For those with an older case, press the green button on the right to turn the case on. Keep in mind the green button is only to turn on the case. If you wish to have the updated switch then please contact customer support.