Hi MarciK - if you own a different ISBN than the one you see listed in the EYB Library, first check it isn't a linked book, therefore "hidden". Click the links icon and you will see a list of linked books. Click the ISBN of the one you own and you can add your edition from the book details page.
Another way to find editions you cannot see is to import the ISBN number in Import Books, accessed from the My Bookshelf tab (this feature is only available to Premium members). When you import the ISBN, if the book is already in the Library, it gets added to your Bookshelf. If the book is not in the Library, we try to retrieve the data from Amazon. The book goes into the Library and onto your Bookshelf. If Amazon doesn't have the book listed then we cannot add it at this time. We do hope to add a feature that will allow members to add their own books.