Well probably you needed a FolderBrowserDialog for WPF but there is not a built in FolderBrowserDialog like we know it of the WinForms technologie.
First of all you could use the WinForms Dialog but in my opinion it is better to use the WPF technologies if we are using WPF 🙂

So I was looking for a Control which contains this functionality.
After a few hours of searching and testing I found a library from Ookii, which contains even more than just a FolderBrowserDialog.
Check this out: http://www.ookii.org/software/dialogs/