Hi,
daisuke2, and welcome to the forums.
If you check your laptop's user guide, you'll find the exact procedure, but most laptops have a special key combination on the keyboard to allow the external port to be enabled. On many laptops, especially Dells, I have seen the combination being the Fn key (usually located in the bottom left hand corner) and the F4 or F5 key (If you look carefully, usually in an off color, like purple, or green, or gray, you'll see a couple of icons that look like a monitor (and one looks smaller than the other) separated by a / - that is the correct Fx key to use with the Fn key.
Also, for future reference, you have to remember that when you ask a question, you need to be *very8 specific and include all information possible - for example, I don't know if you're running Vista 64bit or 32bit (not really important), and I don't know if you've ever used an external video display device with your laptop or not, and *most important* I have no idea what your laptop model number is - because with the model number I could download your manual from the manufacturer website and tell you exactly what page to look on to find what I am talking about.
HTH