Function keys with external keyboard
Solution 1:
The only way is to disable system-wide (that includes the notebook's keyboard) the custom control keys, and tell OS X that you just want to use it as normal F1-F12/F16 keys.
Go to System Preferences > Keyboard. Check the box that says "Use all F1, F2, etc. keys as standard function keys"
If you are using a Mac keyboard, there should be a Fn key. On the latest Mac wired slim aluminium keyboard, it is around the home, page up, page down area. On the wireless slim keyboard, it is to the bottom left.
Solution 2:
Silly me. The fn button is below F13.