A: You need to select one of the New XY Diagram or New Ternary Diagram options from the Diagram menu first.  When you have the new diagram window open the Create Contour Polygon utility is the 9th icon down on the RHS toolbar.  Once you have created the contours to your satisfaction you can then use the Save button on the RHS toolbar (near the bottom) to save the contours as polygons in the new diagram which you can then use to classify other datasets.