You can embed a model in any topic or reply. From the editor, just click the Sketchfab button and enter your model's URL.
You can also embed the viewer when uploading to the 3DSquirrel Marketplace; just put your model URL in the 3D viewer URL field.
You can see an example here, with the 3D viewer located after the model screenshots.
For more info, see 3DSquirrel's Demonstration Topic.