I wasn't able to reproduce the problem locally. However a possible reason could be if the toolbox didn't upgrade properly.
You could try the following in order fix the problem:
- Close all Visual Studio instances
- Start a Windows Explorer
- Navigate to %localappdata%\Microsoft\VisualStudio\10.0
- Delete the tbd files located there (note, you would need to have the System and Hidden files visible to see the TBD files)
- Run Visual Studio and try to use RadChart
In case this doesn't help you could also try to start the RadControls_for_Silverlight_4_2011_1_0419_Dev.msi again and choose the Repair option.
Let me know if that helps.
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