On calling a Text Export (via the File menu) a Nakakoji view is displayed. Clicking that view's 'Export' button results in an OS-style folder selection dialog being shown - select an existing folder or create a new one. The selected folder is the location to which export will occur. This location is remembered in the TBX so it should not normally be required to reset it after first use unless deliberately choosing a new location.
Once export starts a dialog appears showing the progress of the export process.