How to live feed video from a Nikon D3200 to a Mac?
Since your camera supports NTSC and PAL video over HDMI, recording video from a mac won't be much of a problem.  You'll just need a small conversion box to get the video in.  For our macs with thunderbolt, we use an Ultra Studio Mini Recorder.  If you don't have thunderbolt, Grass Valley has a product that works over firewire.  Once the feed's connected, you can capture from within Final Cut.  Just open the import window (cmd-i) and you'll see a list of cameras at the top left.  In fact you can probably try this out now, because if your monitor has a camera in it, it'll be listed as a facetime camera.  Select the facetime camera or ultra studio or whatever your input source is, and click import.  When you're done, click stop import.  The recording will show up in your events browser.

As far as taking pictures from your mac, I know there's an app for Cannon cameras that lets you adjust settings, trigger the shutter, and download the image over USB.  Nikon probably has one too, but I don't know for sure.