Look at the screenshot in the first post of the Modify ePub plugin thread
here. That is the screen we are talking about.
However you can't get to that if you can't install it. I've never seen that error before, but you are on a much older version of calibre - perhaps I need to update my "minimum calibre version" dependency. Maybe Kovid will have some insight. I would suggest updating to the latest calibre and then trying to install the plugin again.