Like many of you reading this, I was eager to jump online and download my copy of Windows 8 when it became available to developers yesterday. I pulled down the Windows 8 Enterprise SKU and installed it on my Samsung Series 7 Slate. The install went quickly - I was up and running in about 10 minutes - but I kept getting a nagging message to activate. It had never prompted me for the product key.
If I went through the Control Panel to activate it would generate an error message about volumes and folders, and through the Settings it would simply spin and come back and show not activated. I had to dig around a bit before I found this nice tip that I imagine will help you if you are having the same issue. Be sure you have your key.
From the Start Screen, type CMD to pull up the available command prompts.
Don't tap on the "Command Prompt" option. Instead, drag it slightly (or right-click if you are using a mouse) to select it, and choose the option "Run as administrator."
Now you can type a simple command on the command line:
slmgr.vbs -ipk "XXXXX-XXXXX-XXXXX-etc"
Replace the XXXXX nonsense with your full product key. That's it! This will set the product key for you. In my case, it activated immediately. If it doesn't, you have two options. One is to open the charm bar (mouse in lower right corner, thumb swipe from right, or Windows Key + C) and choose Settings, then "Change PC Settings."
There should be an option for Activation if you are not activated yet. The second is to navigate to your Control Panel, then System and Security, then System, and tap on the available link in the "Windows activation" area (mine shows I am already activated - and don't sweat the fact the screen cap shows that no pen or touch are enabled, because it was taken during a remote desktop session).
Enjoy your Windows 8! For more tips like this, grab your copy of Building Windows 8 Apps with C# and XAML.