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.

You can view your sent items by going to your profile and clicking on the “Messages” link in your main profile box. Your inbox is shown first, with your sent items showing underneath.