Could you please tell me the version of Tableau you're actually using?
You could try to download this BigQuery protocol.tdc, and save it to the \My Documents\My Tableau Repository\Datasources folder. Then try again your process.
Caution: Use a .tdc file only to explore or occasionally address issues with your data connection. Creating and maintaining .tdc files require careful manual editing, and troubleshooting .tdc files is beyond the scope of Tableau Support.