Here is quick guide how to send Private Message (PM)
First of all Click on Private Messages option located in Navbar location is shown in screen shot.
Once you are in Private Messages you will lots of options like Inbox (message sent by others), Sent Items (messages sent by you), Send New Message.
In Recipient Users (put username of member to whom you are sending message) Title (give some respected title to your Private Message (PM) in open white space you can write your message and use Submit Message to send message.
There is another quick way to send Message in post click on username drop down menu will appear just click on Private Message and follow 3rd step.