Visual State Manager conflicts with WPFtoolkit.

how can i get the wpftoolit to work together with framework 4.0?

when i add a VisualStateManager item to my xaml-file in vs 2010 i get the following error:

The type 'System.Windows.VisualStateGroup' exists in both 'c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\Profile\Client\PresentationFramework.dll' and 'c:\Program Files (x86)\WPF Toolkit\v3.5.50211.1\WPFToolkit.dll'

thanks for your help

regards