If you try to add your book and are told the ISBN-13 already exists, this means the title is already on the system.
To prevent duplicate uploads, the system only allows one record of each ISBN, which is why you were prevented from uploading it.
All you have to do in this case is search for the title and assign it to your account. Once it's assigned, you can go to My Books, edit the information and add previews as you would with records you created yourself.
If your book's rights are assigned to someone else and you're not sure why, just click 'Report' under the book image and write a description of the issue. Admin will look into this for you.