It will only work if they have the messaging feature enabled. If they do, it will show up when you click on their screen name - a lozenge shaped tab on the upper right hand-side of the screen. Click on that and a messaging screen will come up.

Click on the profile, there should be a message tab on the top right of their page. If there isn't one, they have set their profile to not receive messages. In which case they need to go to their settings, click on advanced settings and check the boxes for people they are willing to receive messages from. Hope that helps!

KC