Did you just install Windows 8 Enterprise on Your Surface Pro?
If Your machine is not domain-joined or you do not have a KMS server, you can activate Your enterprise installation With a MAK-key. This is usually if you have a MSDN or Technet key and want to test out something.
Right Click on Command Promt and Select “Run As Administrator”
slmgr.vbs /ipk <windows key>
Your machine is now activated and configured as a KMS (Key Management Service) host.