You can send messages to someone by clicking the Send Message link located on their profile page under Connect. You can reply to messages either from your profile page, directly on someone's profile page, or from your Inbox.
In your Inbox, all unread messages will appear in bold. To respond to a message click on the message, and click Reply.
Please note: If you send a message to a girl and she doesn't reply within 2 days, you get your credit back.