How to record live video from nikon d3100?

I have a nikon d3100, i would like to record video straight from it to my macbook, like a webcam to iMovie, or atleast where its in sync with an external mic

Technically you can record "live video" with your camera

However, you are asking about tethering your camera to your computer and there are only a few cameras that have that capability, notably the Nikon D4, D800(E), D600 and now the Canon 5D, Mark III with the newest firmware installed.

This is done via the HDMI port of the camera and the video can be sent to any suitable HDMI enabled device. Control of the video via HDMI is not an option at this time, but may be in the future, but you will have to wait for it and then be willing to buy such a camera.

Recording using an external audio recorder will require you to use a motion picture "clapper" so you can sync the sound later in post.