Have you downloaded and installed the Adobe Digital Editions (ADE) program from the Adobe website on your computer?
http://www.adobe.com/uk/products/digital-editions.html
The normal sequence is to use that to aquire the full copy of the book, and then to transfer it to your Kobo.
Digital Editions has help to show how to do it, and I think the Kobo probably came with some help too,
but if you can't get any further post again and we will try to help.
Try to say exactly what you have done that worked, what you were doing when it failed, and what the message was.
It sounds as if RealPlayer has decided to pretend it can open your files and jumped in to try, then realized it can't handle them.
