Your best bet, I would say, is to get a dent puller. I have heard of people using plungers to fix dents in their cars, but because your dent is on the edge and a seam that would not help you. After you have pulled the dent out or as best you can, you will more then likely not be able to get it out all of the way and there will be a small indication of the dent, the paint will crack and it is then your job to sand down the damaged paint to the smooth metal and prime the metal and repaint the area and try to blend in the new paint with the old paint of the cabinet. This can be a pain in the ass and time consuming, but if you take your time then it will end up looking good again. Well those are my two cents.