Mirror lock up nikon d3100

Mirror lock up nikon d3100 help?

Added (1). how do you you take pics using mirror lock up?

Added (2). i go to Menu, select Mirror lock-up and select Start, and this is waht it says:
When shutter-button is pressed, the mirror lifts and shutter opens. To lower mirror, turn camera off.

than i press my shutter button and it takes picture, but Lcd screen turns black.
I press my shutterbutton again and it says: 'Mirror raised'
I had to swich my camero off

The feature you are playing with is for sensor cleaning. This is all explained in your manual on page 182 under manual cleaning.

Mirror-up mode is only found on the higher end cameras like the D7000, D7100, and full frame models.

Taken from an article at dpreview.com: "Simply enter live view to flip the mirror up, then release the shutter a couple of seconds later when any vibrations have died away."