wpf-4.0

WPF DataGrid not appear in Toolbox?

I am working with Visual studio 2008. I want to work on WPF DataGrid control. I downloaded WPF toolkit and install it, and then added toolkit reference in my project, but it does not recognize Datagrid control, and does not show it in Toolbox, I even try to add it in toolbox through choose Item.. but when I click WPFToolkit.dll, it giv...