Click on the person's user name who you want to send a private message to. A message box will appear. On the top right hand corner is a little green box that says "message." Click that and you will be prompted to type like you do on a regular post.
Keep in mind although no one on the forum can see this message, I am convinced the moderators can. So just be sure and follow the same rules as on the public forum.
Best of luck on your trading.