Extended WPF Toolkit - How to add reference to project?

First, reference those dlls in your project.
Right click on References in the Solution Explorer and click Add Reference, now browse and add the two dlls.

Second, build the project once to enable intellisense in XAML for the newly added dlls.

Third, in your XAML file add the following namespace

    xmlns:wpfTool="clr-namespace:Xceed.Wpf.Toolkit;assembly=Xceed.Wpf.Toolkit"

Tip: Type xmlns:customName="wpftool" and you should be able to see the intellisense list all the relevant namespaces.

Heres the XAML code:

<Window x:Class="WpfApplication1.MainWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        xmlns:wpfTool="clr-namespace:Xceed.Wpf.Toolkit;assembly=Xceed.Wpf.Toolkit"        Title="MainWindow" Height="350" Width="525">    <Grid>        <Grid.RowDefinitions>            <RowDefinition Height="*"/>            <RowDefinition Height="*"/>        </Grid.RowDefinitions>        <Grid.ColumnDefinitions>            <ColumnDefinition Width="*"/>            <ColumnDefinition Width="*"/>        </Grid.ColumnDefinitions>        <wpfTool:DateTimePicker Grid.Row="0">        </wpfTool:DateTimePicker>    </Grid></Window>

Installation Extended Toolkit using NuGet

Install NuGet  Open your Visual Studio.  Open your solution/project.  Open Tools menu, select Library Package Manager and select Package Manager ConsoleRun the following command "Install-Package Extended.Wpf.Toolkit"      

then try you will get the namespace.

NuGet package page (available since 17 June 2016)

Graphical Way to add the controls in VS 2013

1. First, reference those dlls in your project.
2. Right click on References in the Solution Explorer and click Add Reference, now browse and add the two dlls.
3. Right Click to toolbox > General Area and Choose "Add Tab" - Give the name to the Tab say "WPF Toolkit Controls"!
4. Now Clcik on Choose Items..It opens choose toolbox Items dialog box...!
5. In WPF Component tab - Now Click on Browse button and locate the downloaded dll and click on open
6. WPF Component Tab will show all the control
7. By Clicking on OK "WPF Toolkit Controls" will populate all the controls of the WPF Toolkit
8. Now Drag and Drop the Required Control - It will automatically add the namespace reference required.