Your car should have a vacuum operated central locking feature and two keys, one Master Key for the ignition, doors, trunk and glove box which has a square shaped black plastic head. The other is a Valet key, and it cannot unlock the trunk or the glove box, will work on the doors and the "ignition" so you can give it to the valet and feel secure the stuff in the trunk is not immediately available to him. The Valet Key has an oval shaped black plastic head on it. Usually the car is delivered with two Master Keys and one Valet Key. I never had use for the Valet Key as I don't frequent fancy places with valets.
If the trunk was locked with the Master Key and removed from the central locking system (the trunk can be locked with the central locking system, or be put in an independent mode and locked so the valet key concept works) and you lost the Master Key, you are out of luck unless you wish to break the lock as dculkin described. Otherwise, you have to go a Benz dealer and order a new Master Key. I believe the VIN number is all they need.
Otherwise, all you have to do is insert the master key, and turn it 90 degrees and remove it, and the trunk will be back in synch with the central locking system. If you have a vacuum leak, start the car and let it run to charge the vacuum locking system and once the trunk is unlocked by the vacuum system it will stay unlocked, unless you lock the doors before the vacuum dissapates. Good Luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)