This is how I got my foot stuck in light modding...
Go find the files in the original (un-modded) game for the map...
(data/menu/gui/units)
Create a folder in your mods folder (assuming you have JSGME)
(Map Icons!!!)
re-create the file structure to the affected folder
(data/menu/gui/units)
copy the icons there
(click, click)
enable it and go try it out!
or just erase "units" from the OLC file structure in the mod folder and re-enable...