There are two ways to do this.
Remove the right rear seat cushion, and seat back. You will find a lever, that is the manual release for the trunk.
The other way - used in a complete / full emergency where the release lever is broken - is use a large suction cup, such as used by body repairers... a "dent puller" type device... sucked onto the taillights and believe it or not, they will pull out. This is a tech tip from a Bentley employee I know - I have to say, I have never had to do this personally - but its there if you want to do it ! ;)This will provide access to the main battery, and start battery.
another option is to try an charge the battery up
you should be able to do this via the cigarette lighter / accessory socket. You'll need the appropriate attachment for your charger but car accessory shops sell these- Alternatively if you can't get into the car/ boot the other way to do this is jack the car up so you can get under the front.
Locate the starter motor and the large power cable that runs to it - connect the positive battery charger lead to this terminal
then connect the charger negative lead to the metal of the engine block
This should complete the charging circuit and get some power into the battery allowing you into the car