Add files to your project by dragging them into the list view. You can also use the 'Add Files' button or menu item (under the File menu.) Acceptable file types are text or HTML files. UNIX .mbox files should also work, but haven't been tested.
Generate text from your source files by clicking your mouse on the 'Generate' button in the project window, or use the 'Generate' item in the File menu. With the 'Output HTML' checkbox checked, text will be output and saved in HTML format; with it unchecked output will be in plain-text format.