New Post: Spinner; Could not load file

I've successfully used the DateTimePicker in another project. But can not use the Spinner control.

I added the Nuget package to my project.
And added the namespace:
xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"

and my xaml is just:

<xctk:ButtonSpinner/>

The designer looks fine but when I run my app I get the following error:

Exception thrown: 'System.Windows.Markup.XamlParseException' in PresentationFramework.dll

Additional information: Could not load file or assembly 'Xceed.Wpf.Toolkit, PublicKeyToken=3e4669d2f30244f4' or one of its dependencies. The system cannot find the file specified.