I think I'm too stupid to figure out the latest changes to the file... I've just downloaded a physics tool pack for Sketchup which would be ideal because I'd theoretically be able to actuate the model I've already made, but so far, after 5 minutes of poking around, I haven't had any luck with the new tools.

EDIT... So I spent another 10 minutes and decided SketchyPhysics is a non-starter. It doesn't work for Mac OS. I've only found threads confirming this.