Courtney Smith,
I was getting the same or similar errors just a little bit ago.
Here is what fixed it.
Update Visual Studio Web Express 2010 if needed.
Make sure that Silverlight 3.0, 4.0, and 5.0 are installed from microsoft.
Download and Install the Silverlight 5.0 SDK from microsoft.
Download/Install the Silverlight 5.0 toolkit for Visual Studio web.
Go to the ESRI website and make sure that you have the Silverlight Viewer 3.0.
Then download/install the ESRI Viewer for Silverlight 3.0 SDK.
Once I did all that I rebooted and opened up the .sln file that comes from the ESRI sample download, it all loaded great.
I edited the code as described in this thread (because its still not correct) and hit build solution, it all built great.
My only issue now is that I cant seem to add or find the "add-in" when going through the Silverlight viewer. I will be working on that issue now.
Cheers
Aaron Kreag