Head over to the COMMUNICATE tab to enter the Discussion Forums (http://kbdn.groupsite.com/discussion/forums) and select the forum you're interested in. Forums are just various areas of interest -- general matters, announcements, buy/sell/trade, hangar space for rent, etc.
If you want to create a new topic (for example, you have an airplane for sale), select the buy/sell/trade forum and then click on the CREATE NEW TOPIC button towards the top of the forum. You'll be presented with an editing screen -- enter a title (for example, "2001 Cessna 172S Partnership" and in the message area, enter the details of your post.
To reply to an existing topic, just click on REPLY TO TOPIC above or below the topic message.
And, don't worry if you mess up. You can always delete anything you post.