Since you say the Fn button doesn't work you have an expectation that it does something. I believe the default function is you can control exposure bracketing by rotation the sub and main control dials while pressing the Fn button. I bought my D300 new, have never programmed the Fn button and that's what my Fn button does. That is the usual default on a couple Nikon bodies like the D300 and D700. The Fn button, however, can be programmed to perform different functions.
Assign one of the functions to it and see what happens. I have mine set to "Disable Speedlight" for those times when I have a hot-shoe flash mounted but want to take an ambient light shot without having to remove the Speedlight or shut it off.
If I remember rightly, the Fn button and bracketing also require the change of number of shots from "zero" to "something" in order to actually do anything. Am I right and if so, have you changed the number to zero at some point?