Another good question regarding Microsoft's latest operating system, and I can answer it for you.
Once you have got done generating your Windows 8 Pro key on the MSDN website, copy it and either keep it copied, or save it into a text file or something similar.
Now in front of your Windows 8 computer, go to the Start Screen / Metro screen and type "cmd", and it will bring up an app list with "Command Prompt" as most likely the only listed program. Right click on it, and click "Run as administrator" at the bottom of the screen. This will open up a black screen, the command prompt, as you can see below.
Now it's just a matter of copying or typing in the following, and then press enter:
slmgr.vbs -ipk 00000-00000-00000-00000-00000
"00000-00000-00000-00000-00000" should be replaced with your actual Windows 8 key. That process should complete fairly quickly, only a few seconds. Next up there is one final command we need to enter, and that is:
Again press enter to execute the command. Now Windows 8 is activated with your MSDN TechNet key. Some users have reported that you need to restart your system for the activation to come into effect, but during our testing, we didn't need to reboot.