for your Dropdown, put tcodes in value field.
Then all you need in the script is this line:
session.callTransaction(newKey);
no need for switch case..
then attach this script as onSelect event for your dropdown.
What is newKey?
Please read Tamas's link below where Steve explains
OR
launch Script documentation in your system:
http...../sap/bc/personas3/core/resources/doc/PersonasScriptingAPIDoc.html
lastly, please learn how debugging works in scripting:
Debugging options in SAP Screen Personas 3.0 - SAP Imagineering - SCN Wiki
Regards,
Sushant