This usually means you used a different Adobe ID to purchase the book than what you registered on the eDGe.

You can try setting/resetting the Adobe ID on your eDGe to match what you used on Kobo/Adobe DE as follows:
- from the LCD home screen, press the "Menu" button, then "Settings"
- tap "eDGe Settings" then "Register This enTourage eDGe Device"
- fill in your name and the email address you used to register with enTourage
- check the "Use a different Username and Password for Adobe" and fill in the email address and password you used in Adobe DE
- tap "Register"

Now you can try adding the book to your Library again (remove it first, if necessary) and see if that helps.