You're confusing a bunch of different things.
First, the built-in file variable is the actual type of file objects. You don't want to use that, you're trying to hide it with the filename you got back from askopenfile().
And file is not a Tkinter variable—neither the builtin, nor the one you're creating, have anything to do with Tkinter.
The reason your code isn't working is that, inside the Load function, when you write file = tkFileDialog.askopenfile(), you're creating a local variable. That local variable hides the name of the global variable of the same name, until the function exits, at which point it goes away.
Your attempt to use self.file is a great solution—except you don't have any classes. If you want to learn about how to use classes in general, and the idiomatic way to use them with Tkinter in particular, that's a great thing to learn, but it's too much to teach in a StackOverflow answer, and Python already comes with a great tutorial.
If you want to use a global variable, you can do that, but (a) you have to use global file, not global fileName, if you want file to be global, and (b) you have to put that inside the Load function, not at the top level. If you do both of those, then that file = tkFileDialog.askopenfile() will now reassign the global variable file, instead of hiding it with a local variable, so the value will still be available once you're done, to any other function that wants to access it.
However, a better solution is to not try to share a global variable. Just return the value, and have the caller hold onto it and pass it into Display. Since I can't see the code you're using to call those functions, I'll have to make something up, but hopefully you can understand it and apply it to your own code:
def Load():
return tkFileDialog.askopenfile()
def Display(file):
id3r = id3reader.Reader(file)
print(id3r.getValue('performer')
f = Load()
Display(f)
