DSLR/NIKON D300/NIGHT SKY SHOTS?

When I try to use the night sky setting in my camera (nikon d3000), it doesn't take the picture, or says subject is too dark. I'm totally new to this so should I be doing something different? Thanks.

Added (1). It will not let me change the settings of the shutter speed, it says that it is not possible to change under the settings, even though i had everything else

Camera film and sensors only capture light.

If you have the auto-focus turned on, the camera can't focus on nothing.

Try this.

With your lens set to 55 mm, manually focus the lens on the moon.

Set the ISO to 250, shutter speed at 1/250th second and with the aperture at f/16

What you will see is the small moon, but with detail. The same sun falls on the moon as earth.

This is a starting point.

You need to go to a completely manual mode. It is where you set ISO, aperture and trigger speed separately. I'm a Canon shooter but looking at a picture of a D3000 there's an M on the mode dial, go there. Manual focus will also prevent the camera from hunting for focus. From there it is patience, practice and time.