When you receive a PM an email is sent to you.
Click on the link and a window will appear.
----------------------------------------------------------------------
This is the window that will appear.
Click on "Read this message in this window" and the message will appear.
--------------------------------------------------------------------
Click the Reply button on either the top or bottom of the message.
----------------------------------------------------------------------------------
The window will change to this view:
---------------------------------------------------------------------------------
Type your message and then hit the "Send Message" button.
--------------------------------------------------------------------------------
That's it, your reply has been sent.