The sample you've found is for the Silverlight 3 which we don't support anymore. In the 2011.Q3 release we introduced new gauge control. It is almost new control redesigned from scratch. Some of its features are similar to the old control’s features but they are implemented in absolutely different way (using different set of properties), some features are completely new. I would recommend you to take a look into the following sample in the our demo:
Pay attention, In the 2012.Q2 release we introduced a few breaking changes in the gauge control. So to get this sample work you need latest (2012.Q2) release of the RadControls for Silverlight .
the Telerik team