Others have posted this inquiry. eBooks are most often protected by a digital rights management (DRM) system to insure that copyrights are not rudely violated. This entails several interconnected facets. Of course you should have the very latest version of ADE 4 installed. The term "activation" appears to be (foolishly) used by Adobe interchangeably with the term "authorization", in the ADE system.
Go to the Help menu and select "Authorization Info". If you haven't authorization then select "Authorize this device", also from the Help menu, and follow the instructions. But this situation is unlikely because on first run ADE normally prompts the user to follow steps to authorize. So maybe you have authorization but there is some inane snag or problem with it (probably Adobe's fault, not yours). So you must do a "Remove authorization" from the Help menu and afterward do a new "Authorize this device". In order to remove existing authorization you have to know your Adobe user password. Otherwise there could be problems. Even if you know your password, I have discovered there can be problems even so. Keep your fingers crossed that "Remove authorization" succeeds because then re-authorization is not difficult. If you cannot remove existing authorization you may have to manually edit your system registry using Regedit, which is a daunting undertaking for most. This is detailed in another thread -- search for "check activation".
The problems I have had today lead me to believe that there could be some serious malfunctions this weekend with the Adobe Activation Server. Though I speak out of turn. So my advice is to do nothing and try again late Monday.
Keep in mind that ADE is a FREE product, so you don't deserve stellar service.
I dunno! 'Best of luck to you!