Assembly reference not found in XAML, but code compiles when referenced in xaml.cs class

I've got a strange problem adding a dll reference. I've got a WPF application and am trying to use the WPF MDI library: http://wpfmdi.codeplex.com/

As stated in the instructions (which are very vague), I right-clicked on references in VS2012, clicked on Add reference.., clicked on Browse.. and added my dll which I downloaded.

Next, I added the following line in the XAML of my window: xmlns:mdi="clr-namespace:WPF.MDI;assembly=WPF.MDI" as stated in the instructions.

However, when trying to add an <mdi:MdiContainer> in the XAML, the following error messages are displayed:

The type 'mdi:MdiContainer' was not found. Verify that you are not missing an assembly reference and that all referenced assemblies have been built.

The name "MdiContainer" does not exist in the namespace "clr-namespace:WPF.MDI;assembly=WPF.MDI".

This is my XAML:

<Window x:Class="QueryBuilder.Table"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mdi="clr-namespace:WPF.MDI;assembly=WPF.MDI"
        Height="300" Width="300" WindowStyle="ToolWindow" ResizeMode="NoResize" ShowInTaskbar="False">
</Window>

This problem has been frustrating me all day. I've tried almost everything imaginable. One thing worth noticing is that I've downlaoded the sample application from the library's site, and I can run it with no problems. Besides this, I have added this to the xaml.cs class of my project: WPF.MDI.MdiContainer d = new WPF.MDI.MdiContainer(); and it compiles with no problems at all.

Please also note that my initial problem was asked here