AC3D 8 works fine under 10.6.8 here
We work hard to provide compatibility with older Mac OSs. This is despite Apple forcing developers to stop supporting older systems and essentially requiring developers and customers to update their hardware. It is becoming more and more difficult to work around though.
Another difficulty is Apple's move to pushing independent developers to release their software via their App store. Whilst you could argue that this is good for security, a cynical view might be for the control that Apple has over the software and for the cut it makes selling it.
Recent OSs simply won't run a program that tries to access anything outside it's app bundle. This is why lots of the AC3D supporting files (e.g. plugins, datarefs etc) had to be moved inside the bundle. You may notice that it's quite hard to open AC3D for the first time - you might need to open it from a popup menu. This is because it's not come from the AppStore.
The compatibility problems you are seeing may be a result of the unzip/untar tools. Perhaps try using a third party decompressor if you have problems. Let us know how you get on. (Sorry about the long message)