Gene,
1. In your surface style, turn on the component for user defined coutours
2. In your "Surface Properties" dialog box, go to the analysis tab and
select user defined coutours from the drop down list
3. Set the number of ranges to 1, and press the little down arrow button.
4. In the table at the bottom of the dialog box, enter you desired contour
elevation.
That is all there is to it.
Cheers,
Peter Funk
Autodesk, Inc.