At the top right you will find an envelope next to the notifications button. When you hover over it, a box will open where you can see the most recent 3 messages, new messages received and a view button.
Pressing the view button will open the chat module. On the left is the Inbox with all the messages and a field where you can search for certain messages / people.
In the large window, from the main screen, there is a large field that contains the messages received and sent and above it a field called "to" where you will select the recipient of your message.