This works for me, from a terminal, in my arc3.1 directory:
$ mzscheme -if as.scm
Welcome to MzScheme v4.2.1 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.
Use (quit) to quit, (tl) to return here after an interrupt.
arc>
What commandline are you using that's giving you the error?