Mode: Object and Pose modes
Panel: Constraints (Object context and sub-context, F7, or Editing context, F9, when in Pose mode)
The Null constraint does nothing! It is an antiquated feature, which will be removed in Blender 2.5.
This constraint uses no target, nor has it any option…
As it does nothing, this constraint also has no Influence setting, nor the related Show and Key buttons.