We're sorry you're running into trouble sharing from Book Creator. Seesaw does not support ePub format, so if you want to share from Book Creator you need to export your book as a PDF or video.
You can export books as a PDF or video from the main screen of the Book Creator app. If you tap the share button below the book on the main screen, you can then choose to Export as PDF or Export as Video. Both of these formats are supported in Seesaw.