There are two ways to resolve this issue.
- Hold down the fn key to use one of the function keys.
- The better way is to check Use all F1, F2, etc. keys as standard function keys. This is found in System Preferences -> Keyboard & Mouse in the Keyboard tab. This setting is available on all systems with keyboards that have the fn key.
Background:
This problem occurs because of a design change in the Apple Aluminum keyboard.