Having replaced my home server with Windows 2012, I found despite having the proper keys, I could not activate it. The product would not in fact let me enter my own key at all (see below).
Clicking activate tries to activate with the key supplied on the media which is blocked from activation. The ‘Change product key’ link, or what is intended to be the change product key link, simply isn’t there.
The following workaround does the trick.
Move your mouse to the bottom right of the screen (assuming you don’t have a touch screen!) to bring up the sidebar, click search, type cmd, right click the ‘command prompt’ on the left, choose ‘run as administrator’. The command prompt window should open after a security prompt.
slmgr.vbs -ipk your-product-key
obviously replacing ‘your-product-key’ with your own activation key. You should get a confirmation prompt in a few seconds.
Again, windows should provide a message saying it’s been successful. Reboot and you are done.