The trunk lock performs a few functions and the workings can get a little complicated. If you have locked the doors and trunk using the central locking feature, does putting the key into the trunk key hole and turning it counter clockwise open the door locks? If so, with the key turned, push the button in and lift up on the trunk lid, and it should come open. If that does not work, remove the key in the vertical position, and cycle the driver door lock, then see if the trunk opens (when the doors are also unlocked).
If that does not work, I think you may have something out of adjustment. Kind of a "Catch 22" situation though, since you have to get the trunk open to make any changes to the way things are working. 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)