click on the persons name and you will go to their profile there should be a box that says private message click on that and say what you have to say. OR
click on the private message button and the box opens up and type the persons name in that yu want to message.
OR
if you are like me and can't remember the name and how it is spelled then just copy the name before you click on private message and then paste it on the name area
hope this helps you*