Look at these files using an editor. Note the dimensions of the cube as reported somewhere near the top of this file. T3D will need to know these dimensions. Now delete all of the header information in the file so that the file contains only the data set. Summarize in your notebook the essential features.
Negative values are places where electrons reside.
Try rendering both the 1.0 isosurface and the 0.03 isosurface simultaneously. This will show you both the molecule frame and a surface where electrons reside. Try many other renderings, this is just a suggestion to get you started. Include multiple isosurfaces, slices and animations. Experiment with different colortables.