Which walkthrough did you check -- MaG's one?
Here's what MaG says about using the camera:
How to take a picture: If an item can have a photo taken, a camera cursor is seen.
Small square - Left click the camera icon on the item and see a small square at the very center and ruler at the sides.
Move that small center square on top or center of the item to be acted on until it turns green.
Ruler - See rulers at the side of the center squares.
Use the mouse wheel to zoom in-out until the ruler at the sides turns green.
Shoot - Click-hold the mouse and release when the outer center crosshairs are all green.
A flash is seen and evidence is recorded. Right click to get out of the camera view.
Which part are you having trouble with?
Here is a link to MaG's walkthrough in case you were consulting a different one: NCIS walkthrough