You have to wait until Microsoft releases the Beta / RTM. It maybe a while until it comes out. The version that release now is a Pre-beta version for developers so they can start building software before the full version comes out.
It also listed in the All Apps version of the Start Screen. Just in case you haven't discovered this yet...Right click a blank area of the Metro Start Screen and click All Apps in the pop up bar at the bottom. Windows Media Center's short cut is listed under Windows Accessories. Have fun!!!
you need to request a key to download Windows Media Center and if you register the key before January 31, 2013, it's free. otherwise, you'll have to buy it: