RadTileList is designed to let the user scroll the tiles left and right (depending on its Width and the number of tiles, of course).
So, you can set its Width to match the number of tiles and center the <div> that holds the TileList (this div should also have the proper width).
Or, you can use standalone tiles and center them (or their container) as if you would center any other element: http://demos.telerik.com/aspnet-ajax/tilelist/examples/standalonetiles/defaultcs.aspx.
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