"diggad's" answer is right on the money.....I've been in the same predicament more times than I can remember. One time, I had disassembled the vacuum plunger mechanism in the trunk and closed the trunk lid not realizing that the Master Lock System (vacuum plunger) was in the engaged/locked position. Needless to say I panicked; I thought I would have to try to get into the trunk through the back seat (impossible). I looked in the Owner's Manual, and there was the answer, just as "diggad" explained. Turn the key to the left, you'll feel a slight click, and push in the button with the key in that position. Sometimes you may have to use a little more pressure when turning the key than you think.
It may be helpful to spray some lubricant (WD40) into the trunk push button to loosen up the mechanism.
Good Luck