If you click this button you’ll be shown the Add Torrent dialog.
You can add a http/https/ftp/ftps url here and Koinonein will download the .torrent file then add it to your torrents.
You can add a magnet link here and Koinonein will fetch the .torrent file through DHT.
You can browse your system for .torrent files and add them from here.
You can add a torrent hash here and Koinonein will fetch the .torrent file through DHT.
From Settings > UI Options if you have Set as default BitTorrent Client checked then when you double click on a .torrent file Koinonein will open it, Koinonein will also open .torrent and magnet links automatically when you download or click on websites.
If you right click the Koinonein tray icon you can select Add Torrent and you will be shown a dialog where you can select multiple .torrent files, pressing F3 will will give you the same option.
You can drag and drop a single .torrent file or multiple .torrent files on to Koinonein and the .torrents will be added.