A couple of projector tips:
- If your computer is hooked to the projector properly and you see only the background of the computer screen (no icons or menu bars) showing through the projector, you need to turn on mirroring in the display menu of the system preferences.
- You may want to have that display menu permanently in your top menu bar. To do this, go to the System Preferences, choose the display menu - check "show displays in the menu bar". This will put a small icon that looks like a monitor in the top right menu bar, that can easily help you to detect displays or easily take you to the display menu in the System Prefs.