When you install Windows 8 Enterprise RTM then the installer will never ask you for a product key, instead it will just use a default key from Microsoft which cannot be used to activate Windows. In order to successfully activate Windows then you need to replace this key with your own and the only way to do this is through the command line (it’s not possible to do this through the UI):
1) In the Metro UI, search for command prompt.
2) Right click command prompt and choose Run as Administrator from the menu.
3) Write: slmgr.vbs -ipk "your key goes here" and hit enter.
Windows should now be activated.