Look under the name to the left of the post.
You'll see number of posts, maybe an avatar, then at the bottom of that section, you'll see two little symbols, one a bit like a little portrait, the other a sort of scroll-like 3-D letter S. Click on that scroll-y thing to send a PM. But unless you're talking living people and/or living people's details/exchanging email addresses etc, I've found there's a lot to be said for continuing to post on the public forum - you never know what sort of connection or information other rootschatters will turn up!
