Hi Malcolm,

Welcome to Nikonians!
As Len stated above, if you want to zoom in on a subject before you capture an image you must rotate the zoom ring on the lens.

If you want to crop an image that you have already captured you can do it in camera by going to the Retouch Menu and using the Trim tool to create a retouched copy of the original image and save it to the memory card.
See page 252 in the PDF version of D7000 Users Manual available at Nikons website or on the disk that came with the camera.
Good Luck and Enjoy your Nikons!

Best Regards,
Marty