I'm not sure if MB's are easily opened by a lock smith, but did you try calling one to see if they can slim jim it open or possibly get you in?
Otherwise you might only be able to do it the other method, breaking the window. UGH I would not want to do that.
BTW, how did you lock the key in the car? I don't think the drivers lock will go down unless the door is closed, though I might be thinking of a different car all together.
Maybe AAA can help if you have em...
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.