I think should import as a path if missing in Dynamo, but not sure it can resolve problem exaclly
import clt
clr.AddReferenceToFileAndPath("<the place to your reference IronPython.Wpf.dll>")
import wpf
it can be facing from two problem :
- Missing Library DLLs shipped together in the time install Revit software
- Conflict library IronPython with another third party library or extension.