If you don't mind having a public reply sent, look at the lower left corner right below where you read this; you'll see a button called "P ost Reply". Click it, and a new window will open with a blank section to write your message in. If you scroll down the page a bit, you can even see what others have written before you.
If you want to make a private reply so the other 10,000 of us can't see it, click on her name where it's underlined and in red, right next to her avatar, in the upper left side of her post. Click on the option that says "Send a private message to Marcia". A new window will open and you can write you message. Don't forget to fill out the title box, or the message won't be sent. When you're done typing, click on "submit" and the electrons will race all over the world and pile up in a heap in her mailbox.
Claude