They are switches on the back of the wheel itself. Basically, if you were to hold the wheel at 9:30 and 2:30, to that your thumbs were resting on the top set of spokes, your index finger would wrap around behind those spokes and be right on the switches. The switches are rockers, similar to the window switches on the W163, but they are concave instead of convex to make them easier to feel with your finger. They rock to the left or right. The transmission can be shifted up and down from either switch. This is a little different if you have driven an AMG model (except the W163) where the left switch is down and the right switch is up.
IIRC, pressing the half of the switch toward the steering column is a downshift, and pressing the half toward the outer edge of the steering wheel is an upshift.
- RODNEY