Symptom
The Internet Sharing preference pane does not allow you to share your AirPort Internet connection to wired Ethernet clients.
Solution
At the sharing computer, drag AirPort to the top of the port configuration list:
1. Choose Apple menu > System Preferences.
2. Choose View > Network.
3. Choose Network Port Configurations from the Show pop-up menu.
4. Drag AirPort to the top of the port configuration list.
5. Click Apply Now.
In the Internet Sharing preference pane, you should now see this message:
"You are connected to the Internet over Airport. This connection will be shared with computers connected to Built-in Ethernet. "