If you upgraded your system from TFS 2010 to TFS 2012 and applied changes to your Team Project as described in my post Using new features in upgraded TFS 2012 Team Projects, you may notice that there are no tiles showing under your Team Favorites section of your Team Project’s Home page in Web Access 2012.
To enable these tiles you need to drag and drop queries into your Team Favorites section of your Work Items page in Team Explorer inside Visual Studio 2012.
Once you do that, and values are calculated, you can refresh your Web Access 2012 page and see the tiles.