Use the "+" button and choose how you want how you want to add the torrent. Personally, i use either Magnet or URL. Both refer to the link location.
Basically, I find the torrent I want and then copy the link location. Then go to Itransmission, press the "+" button, then choose URL (or Magnet if it's a Magnet) and paste the file location. It starts downloading the torrent for me.