I've encountered the same issue.
While searching I found a post about a seperate WPF branch referring to http://scintillanet.codeplex.com/SourceControl/latest.
If you go there and click on Branches, you can see it.
I downloaded this one and build it, then I added ScintillaNET.dll and ScintillaNET.WPF.dll to my WPF application.
But still no luck, when comparing my project with the project I got from codeplex, which worked I'v found that
My Application: Project Properties -> Application -> Target Framework was set to .NET Framework 4 Client Profile (this is not the full .NET FrameWork 4 but a light version)
The Application from codeplex was set to .NET Framework 4. After changing my application to the full .NET Framework 4, all issues where gone.
Namespace I added to my XAML
xmlns:sn="http://scintillanet.codeplex.com"