Unfortunately, as far as I can see, the application has been discontinued and its website no longer exists. Furthermore, I can't find any evidence of a Mac version for Need4 Video Converter, so there's no official way for you to use it on a Mac.
However, if you still have the installer, you could try installing a Windows emulator like Boot Camp or Winebottler on your Mac, then set up Need4 Video through that emulator. You probably won't be able to activate it though.
In case you need an alternative solution, I recommend checking out a tool called Handbrake.
