possibly -
- you don't have the mental ray plugin installed (I think). If this is the case, this can be fixed be doing the following -
1) click on 'window' in the menu bar, then go to 'settings/preferences', and scroll down to the 'plugin manager'.
2) once the plugin manager has opened, scroll down to the plugin named "Mayatomr" (should be alphabetical).
3) If either the 'loaded' or 'autoload' boxes are unchecked, that's probably your problem. Check 'autoload', and close the plugin manager.
- If it both of those are checked already, and that portion of the attribute editor is still not showing up, no idea. Other than that; do lights have mental ray attribute portions? I haven't done too much with lights and mental ray, but most of these kinds of attributes are often defined from inside texture attributes on objects....
Hope this helps (if not, well, I need to learn anyways...)