That's a lower-end computer, but I bet you can still get it running. When launching Wooden Nickel, hold down the Option key, and you'll be presented with a graphics configuration dialog. Try lowering the resolution and the anti-aliasing settings, that should help. Regarding quitting the game, Command-Q will close it at any time.
Wooden Nickel's visual simplicity is deceptive—the game is made up of vector graphics that have been converted to meshes, which results in a more CPU-intensive game than would traditional sprite-based graphics. The upside is that it's resolution independent and takes up far less disk space.
Hope this helps!