Installed Snow Leopard on VirtualBox but can't install guest additions
Solution 1:
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.