Messages are called "Conversations". If you have a message, the little envelope symbol at the top will be highlighted. Click on that and it lists messages to you, for you to click on and read or reply. When clicked on, a message box opens.
To send someone a message, you move the mouse over their Avatar (or tap on their Avatar) and select "Start conversation".
To send someone a message, you move the mouse over their Avatar (or tap on their Avatar) and select "Start conversation".