You can't unfortunately. And neither can Richelle. Not yet! Private Messages are restricted to members who qualify for "Respected" Status - achieved through having been on the forum for some time and accrued at least 75 posts + 150 of what are called "reputation" points. Obviously, the longer you've been here ... and the more you post ... the greater the opportunity to earn points and become a 'Respected Member' ... especially where other members find the content useful.
But you can exchange 'Visitors' Messages' on one another's profile pages.