The Silverlight version of Balder also supports the notion of a Game Class, the only thing different is how one sets up the Game Class for the runtime.
Like in any other platform, you can simply derive from the Game class found in Balder.Core.Execution and implement your content loading and game logic.
The next thing you need to do is use your newly created game class in Xaml.
(Remember to add a namespace declaration to the namespace were you have your Game Class)
<local:MyGame Width="800" Height="600"/>