I don't know the library you are talking about, but I used WPF for creating some report, let's say a minimal report engine, so I think I can give some suggestion to you.
First, learn XAML. Visual studio has a good designer, but I use it just to see what's happening: writing XAML with the good Visual Studio intellisense is much more productive. I can't suggest how to operate with your lib, but with my strategy the document pages are wired by the engine in code, content are produced by user controls: they just live very well in VS designer. I also used an MVVM library to develop in a an code free view way.
So what I learn in y WPF experience: it is a great and powerful environment, you can present thing the way you want, but don't look for a cutting edge designer. The WPF designer is XAML, mastering it is necessary.