ok so I have been doing alot of solo playing and reading since I dont have a wireless adapter for my xbox yet but this is what I know of the situation.. some Spoilers ahead.
I dont think anyone has found a card that can be inserted in that machine yet.. although I read someone say that you put it in the nav table after you shoot it with the jet gun. but from the videos Ive seen that doesn't seem to be the case.
If you are in a game with multiple people and they all have cards one of the characters.. (I think samuel) will have multiple cards by his points.. yet I have also seen videos that show 2 different people with 1 card by each.
after you do the dr. maxis and richtofen EE the tower in the corn field seems to have power lines going to the top as well as power emitting from somewhere else (im thinking either the green lights or the bus). my guess is that, A. there are other cards hidden in the level that no one has found. or B. the correct card is from another level from a future dlc or maybe nuketown zombies..
that is just speculation but the fact that its called a nav card (navigation), and its under a power tower, I think it will be used to take you to other levels later.