The Toolkit is not registered to all Windows Phone SDK assembly location.
It was added to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SDKs\Silverlight for Phone\v4.0\AssemblyFoldersEx\PhoneToolkit1009
but Visual Studio expected it in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SDKs\Silverlight for WindowsPhone\v4.0\AssemblyFoldersEx\PhoneToolkit1009
It was added to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SDKs\Silverlight for Phone\v4.0\AssemblyFoldersEx\PhoneToolkit1009
but Visual Studio expected it in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SDKs\Silverlight for WindowsPhone\v4.0\AssemblyFoldersEx\PhoneToolkit1009