The original message should be in the bottom of the screen when you're composing the reply message.
So when you get a PM and click Reply to this Message, on the next screen you should have the reply composing area, then under it the instant codes, then the reply options (preview, save a copy etc) and under that is the original message. You might need to scroll a bit to see it, depending on your screen resolution.
You can only see the original message on the bottom of the screen when composing the first draft of your reply though, i.e. once you decide to preview your own message, you don't see the original message anymore.
I think this is the only way the board software allows you to see the original message and your reply at the same time.