The error I\'m getting is:
Unable to start debugging. The Silverlight Developer Runtime is not installed. Please install a matching version.
The Silveright 5 Developer Runtime is available within the Microsoft® Silverlight® 5 Tools for Visual Studio® 2010 SP1. It is listed under the Details section on that page.
It depends what version you want - if you want v4 then silverlight.net/getstarted/ has a couple of download links. If you want v3 or before then you may have to ask on the forums.
EDIT: The link is currently titled "windows developer runtime"
Direct links: