Upon first launch, or if your Digital Editions doesn't have an authorized user, you are presented with the setup assistant to authorize Digital Editions.
Enter in your Adobe ID and password to authorize your copy of Digital Editions. Make sure that this Adobe ID is the same ID that was used to purchase your eBooks. Also, if you are syncing an eReader device to Digital Editions, make sure that it's the same Adobe ID. For more information regarding creation of Adobe ID, see Adobe account, password, and login FAQ.