Do you mean you want to change your CAD model's parameters with arduino instead of using your mouse and keyboard? Say, if you press button 1, it will scale your model to 200% etc. If your CAD software accepts key strokes as input, you can use an Arduino Leonardo to sense buttons and then send out key strokes as if it were a keyboard. It's very simple.
Well at the moment it's just an experiment. But I'm thinking that I want to create a model that is constantly evolving, then I want to be able to press a button (physically) that will stop the model - which will then in turn be fabricated as a one off item.
OK, if the model evolves, how do you stop it yourself, like pressing a button with mouse, or a key stroke? You can replicate that with Leonardo. Mouse click is tricky since the mouse could be anywhere on the screen.
I still need to research into how it actually works but I would imagine it would be with keystroke or clicking a mouse button. So it will be possible if it's a keystroke.
Arduino is able to send keyboard, mouse and joystick commands to the PC. You should be able to do that with programs that run on the PC too but I'm not sure just how. Even with Arduino feeding keystrokes you have to have the focus window set before starting the code.
Autocad should allow you to manipulate your models but you might need to use Lisp to do that.