Demo instantly crashes

[Mikezila]

Your computer only has half of the memory required. It could be crashing just because it doesn't have enough space to work. It's possible that your problem is unsolvable.

But we can try.

Go to your Applications folder, then the Utilities folder, then launch the app called "Console". Once Console is loaded, hit the broom icon at the top of it. You should have an empty window. Hopefully helpful error messages will popup here when you launch the game.

With Console still open, try to run the game again. Once it crashes, or errors, see if anything interesting is shown in the Console window.

What version of MacOS are you using?

Update:
I just went and tried the demo on a friend's eMac, and from the errors in the console it looks like the game is crashing because the video card does not support the shaders needed for the game to work. Sorry to say it, but you're not going to be able to run the game it doesn't look like.