Click on the blue username link (try this on your own profile, if you want). When the user's profile pops up, click on "Contact Xxx" (where Xxx is the username) At that point, you can send a message to the user's e-mail address of record.
Edit: You can find a user's profile by typing their user name in the search field in MP (on your desktop or laptop). If you don't know their username, it becomes more difficult. If their username is their first and last name, it's much easier, but only a percentage of users do that.