As some of you might have noticed, Windows Media Center is disabled by default in Windows 8 Release Preview. Here’s how to re-enable it according to the Microsoft FAQ:

  1. 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.)
  2. Enter add features in the search box, and then tap or click Add features to Windows 8.
  3. Tap or click I already have a product key.
  4. Enter this product key: MBFBV-W3DP2-2MVKN-PJCQD-KKTF7 and then click Next.
  5. Select the checkbox to accept the license terms and then click Add features.Your PC will restart and Windows Media Center will now be on your PC and the tile will be pinned to the Start screen.

http://windows.microsoft.com/en-GB/windows-8/faq