If you want the moment of inertia values about a particular axis, set the WCS to represent the axis (or axes) that you are interested in. Use the measure body command and look at the values reported about the WCS.
A pricipal axis is an axis about which there are no off axis components of rotation. In other words, the rotation about this axis will be "balanced".
The response above applies to rotational motion of a rigid body (due to the references to moment of inertia and pricipal moment in the thread title). However, the "J" in your equation and the mention of "neutral axis" leads me to believe that you may be talking about a "2nd moment of area" or a "polar moment of inertia", both of which are entirely different. If you would clarify which type of moment that you are looking for, we can provide better answers.