Run the command prompt in admin mode.
Run the command as shown in the image
slmgr.vbs /ipk <your key>
Hope you got it solved.
A preview for the windows 8.1 a update for all windows 8 users is officially released from Microsoft
You can download it here
It is a preview and install at your own risk Microsoft recommends to backup your windows 8 before you update so that you can rollback anytime.
If you are running windows Enterprise http://windows.microsoft.com/en-us/windows-8/iso-preview its not available yet you will have to wait.
Wait for the final release if you cant wait to get your hands on what are you waiting for ??