how do we help the spam system learn from its mistakes? like if a piece of spam gets through, is there an email we forward it to or folder we put it in ?

Yeah, you can do that from within Usermin.

Once you open a message, you'll see "Report Spam" and "Report Ham" buttons (if not, you may need to enable them in the Usermin Preferences).

There's also have the option of manually running the sa-learn program, though that's certainly less convenient.
-Eric