I have created a simple widget with an HTML file that links to an external mp4 video. When trying to add this to iBooks Author, I get the message "main html file is missing". It is not, and as far as I am aware I have followed the instructions on http://support.apple.com/kb/HT5068.
The HTML file runs fine in Safari and other browsers.
I have found one other user with the same problem, but there was no reply, except a reference to the above article.
Solved it - a simple spelling mistake inside the Info.plist - the string following the <key>MainHTML</key> tag contained two letters that had been transposed. (Guess the lesson is use copy and paste to transfer the file name). Hope others with the same problem find their solution is as simple as this.