They probably work as regular F-keys, correct? So you don't mean that they do nothing; you probably mean they do not perform the special functions printed on them. According to the product listing for the +Apple Keyboard+
http://store.apple.com/us/product/MB869LL/A
it requires +Mac OS X v10.5.6 or later+. If you use 10.4.11 as shown in your profile, the special driver for that keyboard is probably not present. Therefore, those function keys probably work as regular F-keys, but not has the special function keys.
Note: You can use System Preferences
Keyboard pane +Keyboard Shortcuts+ pane to assign some functions to the F-keys.