Hey Rebecca,
I advise you to use firebug to inspect the source code of the page and to make modification to the css on the fly.
The module doesn't actually have a #mod_cmc div, becuase you can have several modules on the page and then the ids would conflict. That is why we generate an unique id for each module cmc-signup-204 - where 204 is the id of the module saved in the database. This way you can style each individual module on the page differently.
However if you don't want to style them differently we have a class .cmc-signup.
Kind regards,
Daniel