Open up your filemanager,
Start a new search on the theme directory,
Choose "Find text inside file" or whatever it is,
Search for "IconDir",

It will most probably be inside the theme.rc file though,
Something like:

IconDir "$ThemeDir$Misc"

You need to change it so it points to an actual folder.

OR

IconDir is actually not defined.
In which case you need to find the folder with the icons (lets say its "$ThemeDir$misc\icons\")

and add an evar definition to the theme.rc:

IconDir "$ThemeDir$misc\icons\"

this should fix it...