To activate a Windows 8 Enterprise that was installed manualy use this command in elevated “command promt” to change product key:
First run this command:
slmgr.vbs -ipk “Your key”
Then this command
On earlier versions like Windows 7 there is an “change product key” button, but since the Windows 8 Enterprise usually is deployed using some sort of OS deployment services like for example System Center Configuration Manager 2012 where the product key is pre-defined in the setup there is no need for “change product key” button.