If you want a seamless texture to cover the main face and the bevel, you’re probably out of luck. Illustrator applies textures to one surface at a time, and the main surface and the bevel will be treated separately.
Anyway, to apply textures (vector graphics or bitmap) to a 3D object, place the desired texture into your Illustrator document. Then, drag it into the symbol palette (Window > Symbols) and click “OK” when a dialogue box pops up.
In the Extrude and bevel menu, click “Map art”. Depending on the complexity of the 3D geometry, you’ll have a few or a lot of surfaces to choose from in the top right box. Note that when you step through these, you can see the surfaces highlit in the document if previews are turned on. Apply and scale your texture.