There was a bug in the 4.0 release of s.surf.idw. It aprobably will work if you
specify the site file on the command line instead of letting the program prompt
you for a name. If you find this to be true then check for this line in main.c
and see if they are the same:
parm.input->gisprompt = "old,site_lists,sites" ;
What you'll probably see is that "site_lists" is mispelled in your version.
Michael