O.K. I really haven't fooled around with it much myself but when you're importing your model it's also importing the flat texture map which is what you paint on and then when you save it you can then view it on your model with Photoshop. In other words you can't paint directly on your model in a 3d view ala BodyPaint3d. At this point Photoshop's 3d capabilities are very rudimentary  and mostly only truely useful for compositing a 3d model into a 2d scene.