I have installed Silverlight 4 Toolkit - April 2010. I tried to drop a component in a xaml. But I got an error,
"Unknown namespace http://schemas.microsoft.com/winfx/2006/xaml/presentation/toolkit". I searched a solution for a while, but no luck at all! Finally, I got the cause I forgot to install Silverlight SDK4! I am so silly!
After I installed it, that works! Please remember set your visual studio project to use Silverlight 4 too!