Enable function keys on portable Macintosh computers
By default on OS X, the F1–F12 keys are assigned to Mission Control, hardware controls, such as brightness, audio volume, numlock, and so on. Therefore, you must press the Function (Fn) key with the F-key to invoke F-key commands in software applications.
You can change keyboard preferences so the F-keys work in the traditional manner, without pressing the Fn key. When this setting is activated, you can press the Fn key with the F-key to activate the hardware commands.
Enable traditional function keys
In the Keyboard pane of the Keyboard preferences in System Preferences, select the “Use all F1, F2, etc. keys as standard function keys” checkbox. Uncheck the box to return to the default setting.