I am new to WPF. I want to use datetime picker. I have downloaded the \"Extended WPF Toolkit - 1.9.0\" from below site.
http://wpftoolkit.codeplex.com/releases/view/9697
If you want to stick to the method you are currently using, right-click on your project then choose Add Reference. Go to Browse and locate the dlls you downloaded.
or
Download NuGet here. It's much easier to install third-party libraries.