First you need to make sure you network location is set to Home. Second, Windows Media Center only works with Media Center Extenders, such as Xbox 360. DLNA is supported with Windows Media Player 12. To use this feature, open Windows Media Player. Right click the item you wish to play and select Play To. You should see your DLNA device as an option. Click the device and wait.
Here's a more extensive post that describes the full process: http://blogs.msdn.com/b/e7/archive/2009/05/12/media-streaming-with-windows-7.aspx