How to record video directly to my hard drive i have a nikon d3100?

I need to record video directly to my hard drive I'm doing a video tomorrow but I don't have my sd card, is it possible to record directly into the computer (I have an iMac)

Not directly. Your Nikon has an AV out for viewing on a television, so that could go to a computer video capture card (ala EasyCap) but that would not be HD. Or you could run the HDMI out to a computer HDMI capture card for the same purpose for HD. But either way, just stopping off at the drug store and picking up a spare SD card is the best solution.

I'm afraid it is something that is impossible with that DSLR camera.