Why thank you, that's a rather old MOC, I hadn't expected anyone to comment on it! Feel free to explore my Brickshelf, if you haven't already. My username is Caracal.
PM stands for Personal Messaging, and it's sort-of like a mini email within the forums. If you want to talk to someone privately, simply click their name and click 'Send Message', then you can compose what you want to say.
When you click in a forum section you want to start a topic in (for instance, 'Sets') in the top right corner, above forum options there is a 'New Topic' button. Just click on it and start the topic!