Symptom
The Internet Sharing pane says "select an option below," but there are no options.
Solution
This occurs when only one network port is enabled, and that port is your AirPort card or modem. This is because you cannot simultaneously connect to the Internet and receive client connections (to be shared to) over either one of these ports. You must share between them, meaning at least two ports must be active. Ethernet is the only port over which you can simultaneously make and share an Internet connection.
1. Choose System Preferences from the Apple menu.
2. Choose Network from the View menu.
3. Choose Network Port Configurations from the Show pop-up menu.
4. Select the checkbox of each port configuration you wish to use.
5. Drag the port used for the active Internet connection to the top of the port list.
6. Click Apply Now.
7. Choose Sharing from the Show menu.
8. Click the Internet tab.
You should now have options available. The actual number of checkboxes will depend on the hardware configuration of the computer.