often times on laptops you have to use a function command on the keyboard to turn on the external video port.
On my Gateway it is the blue FN key + the f4 key. When I hit these 2 keys at the same time the laptop screen turns off and the projector turns on. If I hit them again both screens come on. And if I hit it a 3rd time the laptop screen turns on and the projector turns off.
Perhaps your dell is similar. Look to see if you have a little montior icon on one of the f keys. It will be printed in the same color as the FN key in the bottom left of you keyboad.