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Sorry I didn't see this sooner. Since it's Canon-specific it might have got more responses if it had been in the Canon forum.
Have you tried it not mounted on a camera?
Is there any letter at all in the upper left corner of the LCD, like an "E," or is that position blank?
Have you been able to do it before but not now, or is this model of flash new to you and you've never been able to get it into ETTL?