Trying to hook up a second monitor

[Joey.E.CockersMommy]

Not sure if anyone can help but I am trying to hookup a second monitor to my computer

I have a dell dimension xps - the monitor I am trying to hookup is a multiSync 4FG by NEC its an older monitor-

I can see where I think it should be plugged in on the tower - actually its the only place it will logically fit - both plugs are the same shape and there is screws on either side of the plug which has about 13 prongs sticking out of it with three prongs missing. ???

The matching plug in on my tower also has prongs sticking out of it.
Its like it almost fits but somethings not working almost like I have to male plugs or something - its obvious that once the plug is in I have to screw it in but I cant get the plug to go in. I tried to take pics but they turned out really bad.

oh yes and the user guide to our computer is locked in brief case that we forgot the combo too.