You have to find her profile page. Go to the search bar/magnifying glass and punch in her name, then click on what comes up. You can make the message private by clicking the little box next to the message space. I wish they'd make it easier to find a user profile than to use the search feature. Or maybe I'm doing it wrong, I don't know.
The option to message will be there, unless they have it turned off. 🙂