I have a macbook with another monitor. But I couldn't connect USB I had to buy a Mini-DVI to VGA.
To connect an external display, you will need one of the following adapters.
Mini-DVI to DVI: Use this adapter for an advanced digital monitor, such as the Apple Cinema Display, that uses DVI video.
Mini-DVI to VGA: Use this adapter to connect your MacBook to a standard analog monitor, projector, or LCD that has VGA video cable.