The Fn key is not a key that is sent from the keyboard to the computer. It is actually used to change the meaning of keys on the keyboard (such that the keyboard sends different scancodes when the key is pressed with Fn held down).
You should be able to easily write a simple Windows Forms program that listens to key presses and shows you what has been received. Register for the
KeyDown event. The KeyEventArgs your handler receives has various properties that you can inspect to see what is being received.
The FN key isn't a real keyboard key like the other modifiers (shift, alt, ctrl) nor is it a function key like the F1 - F12. The FN key is a hardware only key that simply tells the hardware to send a different keystroke while the FN key is being pressed.
So, you are going to have to adjust the brightness manually like "m-y" stated in the comments or through some other mechanism.