You need an interface device that connects to the "composite video source" and then to your Mac's USB port. Then you need software that understands that input and displays it in a window on your screen (or full screen).
I have the device shown here, called +XLR8 XtraView+.
http://daystar-store.com/videoin.aspx
It comes with software for doing video capture, but I mostly use that software to display the video output from my TiVo (which is just an RCA-type cable) on my screen in a window. At the time, it was the least expensive option I could find, and it works well. It basically makes that window on my screen into a TV. Note: This particular product is only for standard def, not HD.
There are some other low cost options I see now from Elgato and Miglia. If I was getting one now, I'd probably looking into the specs on those products, for example
http://eshop.macsales.com/item/ElGato/10020415R/
http://eshop.macsales.com/item/Miglia/TVM041/
You can see other options from those companies on their respective web sites.