You were supposed to leave the Mac's airport settings alone. That is your wireless device. You were only supposed to modify your ethernet settings.
That tutorial presumes you connect the Arduino to the ethernet port on the router, not to the computer. That would make it much easier.
edit: The ip subnets are supposed to be different. The airport device should be like 192.168.0.2 or 192.168.1.2, The ethernet device should be 192.168.2.1. The Arduino shield should be 192.168.2.2. This is not a bridge. It is a separate localnet network using the Mac as a router.