How to you keep the image after using the payback zoom in/out button on Nikon D3100?

The zoom is brilliant but have been unable to keep the zoomed in pic, usually you can take another photo and keep it, can you do it any other way

Not that I know of. The zoom is only on the screen and doesn't effect the actual image that's recorded.
You can only do that with software.

You are zooming in during preview. To make the zoom stick, zoom in first before taking the picture. If the pciture has already been taken and you have no chance to reshoot it, crop in the computer.