Digital Editions only supports ePub or PDF files, so you will need to convert what you have into those formats. There are plenty of conversion options around, and if I were to stick to the company line I would suggest that you look at getting a copy of Acrobat or InDesign to do this, but most likely that would be overkill for you.
A better (and free) alternative that you may want to look at is Calibre which you can find at http://calibre.kovidgoyal.net/
THANKS A TON! CALIBRE CONVERTS INTO EPUB IN SECONDS. BURGOMEISTER HAS
PROBABLY THE BEST COLLECTION OF MODERN FICTION EBOOKS.WITH ADOBE
DIGITAL EDITIONS AND CALIBRE IT WILL BE A PLEASURE TO READ SOME OF MY
FAVOURITE AUTHORS LIKE NABOKOV,BORGES AND NERUDA.