[plt-scheme] How to have a .PLT distribution compiled on installation

I'm a bit confused about how to package up a library for distribution.
Following instructions in Chapter 6 of the MzScheme Compiler Manual,
Creating Distribution Archives, I type:

    mzc --at-plt --plt modulus.plt collects

where "modulus" is the name of my library and "collects" is a
directory containing a "modulus" subdirectory with my .ss files.

collects/modulus/info.ss is simply:

    (module info (lib "infotab.ss" "setup")
        (define name "modulus"))

...I don't know what else should go in there, if anything.

I notice that other .PLT distributions that I've installed end up with
a "compiled" subdirectory, and that, for example, when I install
SchemeUnit, I see:

    setup-plt: Compiling .zos used by schemeunit
    setup-plt:   in /home/andrew/.plt-scheme/209/collects/schemeunit/
    setup-plt:   in /home/andrew/.plt-scheme/209/collects/schemeunit/gui/

However, when I install my modulus.plt, the .ss files are unpacked,
but nothing is compiled.

What should I do?

Thank you,

Andrew Wilcox