You'll need to obtain your Server Key Management Service (KMS) key from Microsoft’s Volume Licensing Service Center.
If you have KMS serever on your domain you need to install that http://support.microsoft.com/kb/2885698 updates to your KMS server. Just install that and it need restart don worry.
Installed that update your KMS server and enter your KEY whit that command:
slmgr.vbs /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
You can then see the stats on how many KMS clients are activated, as well as checking that you're ready for 8.1 and R2 with the command