^ thats only if you want custom icons. you dont want to do that for every .torrent file on your HDD (if you have many) or every one you download. XTorrent should have its own .torrent icon.
cmd+I on the torrent file, open the "Open with:" arrow, change it to XTorrent and then click "Change All
". or the easier way option+right click on the .torrent and choose "Always open with
-> XTorrent".