Just click on the person you'd like to PM and click "Start a conversation" or
It's that image right there.
Thanks, but the thing is that this is a non-typical way to have this function, which can make it difficult for new users to understand how to use this functionality. I do believe that this website would do better if the term 'send a message to userx' or 'personal message(pm) user x' would be made clear (instead of conversation, which sounds like I want to start an open conversation). I would recommend changing this
and add '/send pm' or something like that. Just make it abudently clear to us simple users, it will probably increase the %rate of returning customers.
While i'm at it I would suggest that you clean up your menu in general, just use some kind of analytic tool to see which menu buttons are used the least, and get rid of them. A cleaner menu will provide more clarity for your users.
Also, I must say that I really like this site, it's awesome and as far as I know, the only good RPG maker; well done