[plt-scheme] PLT MzScheme - How to use "--main"?

Hi!

Can someone please explain me how to use MzScheme to run a Scheme program  
with the "--main" option?
For example, I have this program:
(define (main)
   (format "test"))

When I run it like this "mzscheme test.scm --main", I was expecting it to  
print "test", but it doesn't.
Am I missing something?

Thanks,
-- 
Márcio Faustino