It works now!
The app is able to scan torrents in the folder defined in the Preferences (maybe you could make a better guess of the default folder to me it Just Work? I had to point it to my personal folders)
Now when you click a torrent link with the browser you get a dialog giving you the chance to "Open with Transmission" and "Save to device". I'm not sure "Open with Transmission" is working right now. The torrent seed gets downloaded (you see the browser dialog briefly) but Transmission is not activated or updated in any way. If you "Save to device" and place the torrent in the right folder, a Transmission dialog pops up (if you have the app open, of course) and then the rest goes well.