I used to try with 3D model viewer extension but failed, even the URL is right. One of my user tried and failed too. Seems the extension has some issue.
But this extension is from ThingWorx Lab, which is not supported by PTC. We cannot check about its source code.
I suggest to use WindChill extension. As far as I know, there is a widget named CreoView after you installed WindChill extension. And this widget can display 3D models.
I'm the original developer of the extension.
The extension is capable of opening various model files. The modelUrl property should point to the obj file that you want to view. For example, if your file is at http://example.com/myobjfile.obj, then you would write http://example.com/myobjfile.obj for that property.
Another way of doing it is to add your obj file as a MediaEntity. Then the link should look like this: /Thingworx/MediaEntities/MY_MEDIA_ENTITY.OBJ