If you find that the keyboard commands are not working, open the Keyboard control panel, click Options, and deselect the "Make F1 through F12 the primary function keys" setting, or press the fn key on the keyboard while pressing the keyboard command.

Figure 1, Options dialog box
For more specific information on adjusting brightness, volume, and muting, see article number 58327: "PowerBook: Using the Multi-function Keyboard".