Installed Snow Leopard on VirtualBox but can't install guest additions
From Virtualbox Manual : Chapter 14: 


  14.2. Known Issues
  Mac OS X guests:
  VirtualBox does not provide Guest Additions for Mac OS X at this time


What you are trying to run is the script for Linux distributions. That does not work on OS X due to numerous differences in the system, despite a similar-ish base.

From the content of this, I wouldn't hold out for such support in VirtualBox any time soon.