Here are a few more details.  This is kind of tricky because Chimera focuses on 3d images, not 2d plots.

  Use volume dialog menu Features / Planes to show one plane of your map, switch to surface style, use volume dialog menu Tools / Surface Color and color the plane by "volume data value”.  Now we make the contour lines by making a copy of the map, volume dialog menu File / Duplicate, show the same plane, set style to Mesh, turn off the “Cap high values at box faces” option (Features / Surface and Mesh Options), add more contour levels using ctrl-click on the histogram, increase line thickness (Featues / Surface and Mesh Options), color all levels black (command "color black #1”).  Since the planes are at exactly the same level the contour lines are probably hidden.  To fix this move the contour plane slightly forward with command “move z 1 model #1 coord #1” (move map #1 by 1 Angstrom along the z direction of the coordinate system of map #1).  I did this on a ryanodine receptor map, emdb 5014 (plane 89). Here’s the result.

Tom