When using a projector with a computer, you have two options for displaying, mirroring and spanning. Spanning uses the projector as a secondary screen, while mirroring shows the same image on both screens. A majority of the time you will want mirroring turned on. You can turn it on with the following steps:

1. Go to the Apple Menu, select System Preferences.

2. Select Displays.

3. Click on Arrangement

4. Click on Mirror Displays

5. And your done!