There is a trick to accessing the weights.
2 Open the Keyframing Tools rollout and click the Manipulate
SubAnims button.
The Motion panel now displays additional rollouts for Position List, Scale
List, and Rotation List.
3 Scroll to the Rotation List and select the Layer->Noise Rotation.
Now you're ready to animate the Weight field.
4 Turn on Auto Key
5 Move the time slider to frame 70.
Using Controllers with Biped | 835