Why is XAML(WPF) not searching in my XML namespace definitions for the controls?
Althougth the tutorial worked for Silverlight, I believe it may be inaccurate for WPF. See accepted answer here.
I have a separate assembly with custom controls in WPF and I'm able to use the XmlnsDefinition attribute without a problem. I think the problem lies within the definitions need to exist before the assembly is built. So even though controls will show up in your designer, it can't be built because the compiler needs the definitions while compiling the XAML in BAML which are built into the assembly at a later point.
Edit
I created another WPF user control library and added the same XmlnsDefinitions you used, added the reference of the user control library to my WPF application project, used <cui:UserControl1 /> in my MainWindow.xaml and had no errors.