Although laptops now outpace desktop computers in overall sales, it is often necessary to connect a laptop to a larger display. Fortunately for users of both Windows and Macintosh computers, monitor connections (VGA and DVI) are universal and platform-independent. Apple's laptop line (the MacBook, MacBook Pro and MacBook Air models) use either a DVI or a proprietary interface, the Mini DisplayPort, but include an adapter that allows them to easily connect to a DVI or VGA display. Mini-to-VGA as well as VGA-to-DVI adapters are inexpensive and easy to find.
Check to see if your Dell monitor has a VGA or DVI connection. VGA is an older technology, but many monitors have adapters for both connection and include two cables: one VGA and one DVI. Likewise, check your MacBook to see if it has a DVI or Mini DisplayPort jack. Locate the appropriate adapter if required.
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Confirm that both the MacBook and the external Dell monitor are powered down. Attach the video cable to the Dell monitor. The other end must be attached to the MacBook, either directly or using the adapter.
Confirm that the adapter and cable are seated properly. Make sure that the connection area is out of the way of anything that might knock it out of place or possibly damage the adapter and/or computer by bending the pins.
Turn on the Dell monitor first and the MacBook second. When the computer is finished booting up, you will see the desktop on both displays.
Launch the System Preferences (found inside the Applications folder) and click on "Displays." A "Display" pane will appear on both the MacBook screen and on the Dell monitor screen. Note that you can configure the resolution of each of these displays separately.
Select a suitable resolution for each of the displays. Then click on the "Arrangement" tab on the pane on the Mac screen. This is when you decide how you want to configure the arrangement of your displays. You can drag the Dell display to either side of the Mac display to extend the desktop, or you can select "Mirror Displays," which is usually enabled when you plan on using the external monitor as the main display.
Drag the menu bar from the Mac display to the Dell display in the "Arrangement" pane if you want to use the Dell as your primary display and have the Mac menus on the external monitor. You can also configure the color on both displays by clicking on the "Color" tab.