If i'm in pd and click on bang, a panel opens which shows all my 'normal' files, but not the /usr/ tree. Those are hidden. I'm still figuring out, if it's possible on osx3 to see all my files instead of the files that osx decided to let me see.
In OS X, when in a file Open dialog and /usr does not show up, type "/usr" in the "Go To" field.