Hello dlhotka!
Thank you for your answer! If you mean mounting the iso in the CD/DVD-settings instead of the superdrive, I can tell you this doesn't work either as I have tried this, too. Sometimes there will even be a further error message popping up which seems to be sent by Fusion but I have never finished reading it because it disappears quickly. I will try making a screenshot of the message. Maybe it would help if you know what Fusion is trying to say?