I downloaded a 3rd party library for a Silverlight app I am writing. When I ran the app I got an error message telling me to add
to 'devenv.exe.config' (Usually in 'C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE').
It seems you also need to edit the 'Blocked' property of the DLL.
See How to: Use an Assembly from the Web in Visual Studio for full details.