Check the PowerPoint Help article "Set up your system to use the presenter
tools with two monitors".
Here's the relevant excerpt:
"To use the presenter tools during a slide show, you must have two monitors
connected to one computer, and have them set as nonmirrored. To use the
presenter tools to rehearse your slide show, you don't need to make any
adjustments to your computer.
1. On the Apple menu, click System Preferences.
2. In the System Preferences dialog box, click Displays, and then click the
Arrangement tab.
3. Clear the Mirror Displays check box at the lower left of the dialog box.
If there is no Mirror Displays check box, both monitors are mirrored and
that setting can't be changed. The same thing will show on both screens
(either the presenter tools or the slide show4. Drag the monitor icons so
that the larger icon (representing your monitor) is on the left and the
smaller (representing the audience monitor) is on the right, and then close
the dialog box.
The presenter tools appear on your monitor, and the audience view appears on
the audience monitor."