You can't the class is not available.
You can attempt to get around it with something like this;
dynamic shell = AutomationFactory.CreateObject("WScript.Shell");
shell.Run("cmd.exe", 1, true);
Note that example most likely won't run in a browser and if you run it out of the browser I'm guessing the trust has to be set to full trust.
From my point of view, if you need to open a command prompt from your application then you shouldn't be using Silverlight.
Hope this helps,
Fredrik