Are you one of those Mac users who generally turns on the F1, F2, keys as standard function keys option in System Preferences? If you do that, then if you want to use your keyboard to control the volume or the screen brightness, you need to find the Function (or Fn) key. (Alternatively, if you don’t turn it on then you need the Fn key to simulate F1, F2, and so forth keypresses).
On the laptop and the wireless keyboards, Apple usually puts the Fn key on the lower-left hand corner, next to the “Control” key. But it’s not there on the full-sized Apple Keyboard with the numeric keypad!
Or is it?
Ha, it’s in the middle of the keyboard between the letters and the numeric keypad, right below the F13 key and above the Delete key!