Well maybe I read it wrong in the manual (wishful thinking?)
I know I read that the camera can fire an external flash with the pop-up flash according to the manual. What I am not sure about then is if the EF-42 is capable of being triggered externally this way. From the way I read your post, I am assuming it can't?
Edit to add: OK - I read through the flash manual again and I see now that this flash does not have this function at all - can only be used on the hotshoe or with an off-camera cord or radio trigger.