What does the red steering wheel indicate on the ml320?

The red steering wheel in the instrument cluster means that the steering wheel is not locked into position. There should be a black lever on the left or right side of the steering column. This is the lever to release when you want to adjust the steering wheel up or down. Push the lever back up to lock into place and the indicator should go out. If it is still on when the steering wheel is locked, then that could be a electrical fault with the sensor.