Hi Rebecca, assuming you can get into the car itself, open the bonnet. There you will find a suitable positive and negative post to allow you to fit a 12v power source, such as a battery charger or power pack. This will allow the whole car to power up and you will be able to pop open the boot, using the interior switch or boot lid button, to put the charger leads direct onto the battery.
Do not try and start the car from a flat battery situation using the under bonnet posts.
I don't believe the secret keyhole will work with a completely flat battery as it still relies on the electrical solenoid to energise the unlock circuit.
N6 JMX