I don't encourage private emails because most questions are more usefully answered here, as someone else generally wants to know the answer to the same question, sooner or later. Also I use the forum myself as a reference, to see if a certain topic has been answered before.
However for genuinely private stuff, you can send a mail message by the forum mail. Click on my name at the top of any of my posts, which brings up my profile, and then click on "Send an email to Nick Gammon".
You need to log into the forum (see boxes in top RH corner). Then the sending mail feature is activated. Otherwise it can't do that as it doesn't know who you are.
I was logged in. I still am. There's still nothing. I even ran it on a different OS in a different browser.
EDIT: While looking through other profiles, I noticed that I can email certain users (e.g. Ophianne) but not others (e.g. Geraint). Is there any chance that you just disabled it for your account?