Taverna startup (installation) directory is the directory where you installed Taverna and where you run it from.
On Windows, if you have used the installer to install Taverna this is the
C:\Program Files\Taverna Workbench x.y.z or
C:\Program Files(x86)\Taverna Workbench x.y.z directory. If not, then this is the directory where you unzipped Taverna.
On Mac OS X, this is the
/Applications/Taverna x.y.z.app directory. To find your Applications directory, click on Applications under Places in Finder. You can browse the contents if the app directory by right-clicking
Taverna x.y.z.app and selecting the Show Package Contents option.
On Linux, this is a directory where you unzipped Taverna.