[plt-scheme] HtDP languages + emacs

I mostly edit and debug Scheme programs from within emacs. I have installed
Quack which allows me to switch between different implementations. However,
I cannot figure out how to run plt programs that rely on the "student
languages."

In other words, Quack allows me to choose between mzscheme and mred, but
neither of these seem to support the following:

(require (lib "draw.ss" "htdp"))
>
(start 300 300)

 (draw-solid-disk (make-posn 100 50) 10 'red)  -> reference to undefined
> identifier: make-posn
>

Does this require me to start mzscheme (or mred) with a specific switch? If
so what?

(Different  variations of this have not worked for me under mzscheme 209 and
301, both running under Ubuntu Linux.)

Thanks for any help.
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