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Hi,
I think something is wrong on the OS level on your Mac, but not sure.

Can you try to find on your Mac this file:

DirectorShockwave.plugin

The file should be in "Internet Plug-ins" folder inside your home Library folder:
~/Library/Internet Plug-ins

If you will find this file - Quit Safari, MOVE the file I mentioned above from the original location to some safe place (for example to USB stick) or trash it, than restart Safari. Now try AutoCAD installation again.

Maxim