The output of StatNonParamTestPDM is in the ITK-Meta format, which has been replaced with the VTK format. The distribution should contain a converted called Meta2VTK that lets you convert the mesh data.

In order to attach maps, such as p-value maps, to the meshes, you need to use MeshMath

MeshMath   -KWMtoPolyData

then you should be able to see the p-value maps etc on your meshes with ShapePopulationViewer

Martin