You can send a message to another user by going to the user's profile and clicking on the “Send message” icon below the user's picture:
Then just fill out the subject and message body fields and click “Send”:
You will see a notification on your profile when you have received a new message:
When you click on it, a new window will open where you can see a list of your messages, with unread messages marked in bold. Just click on the subject to expand to view the message body:
After viewing the message, you can reply to it or delete it using the corresponding links underneath.