There is a few different ways that you can activate Windows 8.
The first way is to do a search for Slui.exe 0x3. Run the Slui.exe 0x3 command in search and click the Slui.exe 0x3 icon.
Then type your product key in the Activation window then click on the Activate button.

The second way is to run a command from the command prompt.
Make sure you are logged in as a Administrator then do the following.
In the search screen type CMD, which is short for command.
Press enter or click on the command prompt.
Then type the following command.

Cscript.exe %windir%\system32\slmgr.vbs /ipk “Your product key”

This will use the serial key you enter and activate your copy of Windows 8 with the key you supply.