On the right hand side of each post is the poster's name. under the poster's name you will see a little button marked PM.Click on this and you will be able to write a Pm under the quote of their post.Alternatively,scroll down to the bottom of the index page,there you will see who is on-line,click on their name and that will bring up profile page where there is a link you can click, at the bottom left hand side,to send them a PM.