The best way to take B&W photos is to shoot in colour and then convert to B&W in post-processing. In-camera, if you're shooting JPEG, the camera will just dump the colour information (which is *not* something you should ever want to do); the equivalent of just hitting "Grayscale" in Photoshop. If you shoot RAW, the colour information will still be in the RAW file, but I keep it on colour; it's such a simple thing to do and do better in post that it's not worth the time to do it on-camera.
If you really want to have the camera do it for you, find the "Picture Style" setting in the menu, and change it to "Monochrome".
Edit: Ah, so the down button does. I wish I could re-map that to something else. I handle all the contrast, vibrance and saturation in post, which is all those "picture styles" give you anyway.