To begin adding torrents to your rutorrent seedbox, click the add torrent button, this is located on the top bar if you hover over the icon the add torrent text will appear.
This will open the following box.
The other option is to add a torrent through direct upload of .torrent file
You might find it more convenient to download the .torrent file from your private tracker and upload to your rutorrent seedbox. If you prefer to do this, click the browse file button and simply browse to the .torrent file you wish to add, and click open. This should add the .torrent file, so now, simply click the add file button.