When all load cases (XX, YY, ZZ, XY, XZ, YZ) have been computed, the principal material axis can be visualized by clicking Principal Material Axes – Strain Equivalence or Principal Material Axes - Energy Equivalence under the Strain/Stress Visualization tab of the Result Viewer.

The axes are best to observe when the structure is transparent. For this, switch off the visualization of the results (View → uncheck Volume Fields), choose the Structure Renderer to Box and set the Transp. Mode to Volume (both options under the Structure tab in the Visualization panel).