If you download Windows 8 Enterprise from TechNet or MSDN and you don’t have a KMS server in your environment you cannot enter your Windows Key via Windows GUI. Normally you don’t have to because Windows 8 Enterprise gets activated automatically by using your KMS infrastructure, but in a isolated Lab environment you may not have a KMS server. In this case you can run the following commands to activate your Windows 8 Enterprise installation. You have the following two options to activate Windows 8 Enterprise with the MAK key:
Command line option:
Run the following command to enter the key
slmgr -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
GUI option
Run the following command to get the GUI to enter the key
slui.exe 3