To install Activator for iOS 8, you’ll need a jailbroken iPhone running iOS 8.x. After that you will have to add Ryan Petrich repo by going to Cydia—-> Sources—>Edit and type the following repo:
http://rpetri.ch/repo/
Once you add his repo, you’ll be able to download the iOS 8 versions of Activator, FlipSwitch, and AppList.
So have you downloaded Activator on your iPhone ?