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 and then do one of the following:
If you need to purchase a product key, tap or click I want to buy a product key online.
You will be prompted through the steps to buy a product key from there and it will be entered for you.
If you already have a product key, tap or click I already have a product key.
Enter your product key and click Next.
Read the license terms, select the check box to accept the license terms, and then click Add features.
If you were running Windows 8, your PC will restart automatically and Windows 8 Pro With Media Center will be on it.
If you were running Windows 8 Pro, your PC will restart automatically and Windows Media Center will be available for use.
The last two sentences are unfortunately wrong. They should read : "......, your PC might restart......"
Not working, cannot find it where indicated, nor in Windows store (a Microsoft support tech couldn't find it either)
windows.microsoft.com/.../feature-packs seems like it should be a good and similar link for this. However, I'm not seeing an option to "add features to windows 8" at all. I am on Windows 8.1.