Ah, well I'm glad I asked about it then. The email that gets sent is a courtesy from the forum software, so responding to it just doesn't work. However, that is a feature that can be turned off so that instead you can just get a message-less notification or even no notification at all. To reply to a private message, you have to do so from the private message section on the forum. Depending on the theme you're viewing, there should be a place to the top right or in the main menu that says 'messages.' That will take you to your message center where you can compose or reply to private messages.
I just doublechecked the Augustine link. She's working for me using Mozilla Firefox.