I'm working on the todo list application in chapter 2. I have it running in the emulator. When I click the center button (amid the arrow buttons), however, the item in the text field is not being added to the list. The debugger does not show that the "onKey" method is being executed. Is the oval button amid the arrows not the correct DPAD_CENTER button?

Check to make sure that the emulator you are using has d-pad functionality.

Open AVD manager, choose the emulator, hit Edit, and check Hardware.

McRae is right, you should give your ADV DPad support, just as follows:
Open your AVD Manager -> Choose a AVD and click Edit... -> in Hardware frame click New... -> and select DPad Support -> OK
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Good luck!