[abenedik]

Now I see where was the problem:
You have changed the WireframeModel reference from the object defined in XAML to another object created in code (mGeometry). This did not change the WireframeModel defined in XAML - it still remained empty.

To fix your code you have two options:

1)
Set OriginalModel property on WireframeVisual3D to your new Model3d:

Code:

WireframeVisual.OriginalModel = mGeometry;

Note that the content of WireframeVisual3D that is defined in XAML is also set to OriginalModel property.

2)
Update the WireframeModel and call RecreateWireframeModel method:

Code:

WireframeModel.Geometry = mGeometry.Geometry;
WireframeModel.Material = mGeometry.Material;

WireframeVisual3D.RecreateWireframeModel();

It is also possible to use the Ab3d.Models.WireframeFactory.CreateWireframe method and add that directly to your scene.

Though I would recommend using the WireframeVisual3D because it gives you more options on how to show wireframe.