Do you mean the switch in the remote?

Press the remote button twice within 3 seconds to pop open the trunk (without unlocking the other doors).

The owner's manual also has a few other things of note:

Valet security locking
The security locking feature of your car allow you to prevent unwanted access to your trunk and glove compartment, while still allowing a parking attendant to unlock, start, and relock the rest of your car. To prevent unwanted access to the trunk and glove compartment do the following:

Give your service key to the parking attendant.
Insert your master key into the glove compartment. Turn it 90 degrees clockwise and remove the key. The glove compartment cannot be unlocked with the service key.
Insert your master key into the ignition and turn it to position II.
Press the trunk lock button in the center console. An LED and a message in the text window will show that the trunk lock has be activated.
Turn off the car and remove the master key.
With the trunk lock activated, neither the service key nor the remote control will open the trunk.
The function can be turned off (deactivated) by turning the ignition key to position II and pressing the trunk lock button again (the LED in the button will go out and VALET LOCK OFF" will be displayed in the text window).

* The position of this button may vary, depending on the specifications of your car.


And don't forget to try the inside door switch for the trunk.