Media Center is not in the Windows 8 Developer Preview. It will probably be in the beta/RC when they are released in the future. Their release date has not been announced yet.
Not all the features are included in the Windows 8 Developer Preview, including Media Center.
From the Building Windows 8 blog:
NOTE: The preview build does not include every feature shown this morning. Shown but not in the Developer Preview release include the Windows Store, Windows Live Metro style apps, and some of the user interface features. The focus of the preview
is the API and development tools for building Metro style apps.
http://blogs.msdn.com/b/b8/archive/2011/09/13/welcome-to-windows-8-the-developer-preview.aspx
