Depends where you downloaded it from. Many booksellers have their own codes and you need to get one from them.
There is alos a generic AdobeID code wihich may be what you want (for example for borrowed library books; in UK at least).
You get that free by registering at the Adobe Site, http://www.adobe.com/ Choose 'Sign In', then 'Don't have Adobe ID' and follow from there.
That works for lots of very varied features on the Adobe site, and you can enter it into Adobe Digital Eeditions (menu~Library~Authorize Computer).