Right-click the root folder and use the "Add" option. If it's greyed out or not visible, then the format does not support adding new folders and/or files.
As you may have read in the initial post or our github, this tool is no longer supported. There will be no bugfixes or support for this software anymore.
The only fixed format supported to export text from Kuriimu is an XML variant. This is done to preserve as much information as possible from various text formats and make them potentially compatible with other CAT tools.
If you want the text in...