See this discussion item for more info: http://snoopwpf.codeplex.com/Thread/View.aspx?ThreadId=236335
Sometimes, neither PresentationFramework.dll ... nor PresentationFramework.ni.dll show up as modules for the process you are trying to Snoop. Add wpfgfx_v0400.dll to the list of modules to use in order to determine whether the application is a WPF application.