To save time when developing Python scripts and add-ins you should try running them using the Python interpreter first, to verify that their syntax is correct. You should also use the console version of Showcase so that you can see error messages along with the output of print statements.
Avoid testing your scripts by dropping them onto the Showcase application window, because changes to your scripts will not be incorporated until Showcase is restarted.
You may find that enabling the sample add-in EchoMessagesCustom.py included in the extras/Interactive folder can make troubleshooting easier because it will output to the console all messages seen by Showcase.