http://www.ehow.com/how_5661153_set-display-macbook-mirror.html
Open the "Apple" menu, located in the upper-left corner of your laptop's display. Click on the option labeled "System Preferences."
Click on the checkbox labeled "Mirror Displays," located in the bottom-left corner of the window.
Click on the red button in the upper-left corner to close the "System Preferences" window.
TIPS
If you have not reassigned your function keys to a different purpose, you may also activate monitor mirroring by pressing the MacBook's F7 key.
You may connect a DVI, Duak-Link DVI or VGA monitor to your MacBook; however, use a Mini DisplayPort adapter designed for the monitor you intend to use.You may purchase MiniDisplay port adapters directly from Apple's website.If you wish to connect an Apple display equipped with a ADC connector, use a DVI to ADC adapter in conjunction with a DVI to Mini DisplayPort adapter; you may purchase a DVI to ADC adapter directly from Apple's website.If you have multiple monitors connected to your MacBook, but only wish to mirror to a single monitor, link the MacBook's display to that of the target monitor. While in the "Arrangement" section of the "Displays" window, click on the square representing your MacBook's display; hold down the option key, drag the square to the one representing the target monitor and release.If the display resolution on the external monitor is higher than that of the MacBook's LCD screen, the monitor will use the lower resolution while mirroring is active.