Are you sure ac.scm is in the directory "C:\Tools\MzScheme\"? I get the same error when I intentionally try to load arc in an incorrect directory. So if you aren't in the directory where you put arc, cd to that directory before calling mzscheme.
(You may also want to note that you want to load as.scm, not ac.scm, because as.scm starts the toplevel. But I don't know if that is part of your problem or not.)
