There are 2 ways you can send a private message to a user:
1) Click on the Message icon in member menu or on profile page. In the Recipient field, type in the username of the user (it may auto populate) you want to send a message to. If the username exists, it will populate as you're typing in the Username. This will help to ensure you type in the correct username.
2) From the users profile you can click Contact.