Unable to load the metadata for assembly 'WPFToolkit.Extended'

You get this message when try to add reference to the WPF Toolkit to a project and build the project.  You will also see there is no reference added to your project and you can't use any of the controls in the Tooklit.

The problem here is when you download an assembly from a Website, it is block by default. All you need to do is, unblock the assembly.

Right click on the DLL and click Unblock button. As shown below. 

Remove and add reference again. You are all set.