Swipe in from the right edge of the screen, and then tap Search. (If you’re using a mouse, point to the upper-right corner of the screen, and then click Search).
Enter add features in the search box, and then tap or click Settings.
Tap or click Add features to Windows 8.
Then tap or click I already have a product key.
Enter the product key, and then click Next.
Read the license terms, select the check box to accept the license terms, and then click Add features.
Just select "already have a product key" and enter the code.