Are you in the Shaders layout (the Shaders button on the top nav bar)? If so, Shaders should be shown in the 3D preview. However if you're in a different layout, or you disabled shaders in the 3D preview either accidentally or intentionally, you can re-enable them easily with the far-left button (just to the right of Reset) on the top of the 3D preview, it looks like a shaded sphere.
If that doesn't fix it then it may be something to do with the distribution itself, or some other problem. But that above seems like the most likely source of a problem.
- Oshyan