[plt-scheme] importing into mzscheme

I am trying to uset set-car! and have figured out that I need to
import (rnrs mutable-pairs (6)):
http://docs.plt-scheme.org/r6rs-lib-std/r6rs-lib-Z-H-18.html#node_idx_1276

But I can't get the import statement to work. My code is:
(import (rnrs mutable-pairs (6)))
(define a-list '(a b c))
(set-car! a-list 'x)
(display a-list)

I start mzscheme and (load "the-file.scm") and receive an error message:
import: misuse of unit keyword in: (import (rnrs mutable-pairs (6)))

What's the secret to getting this to work?

Thanks, Roy