You haven't read the rules yet.
FAQ: Why Can't I Access Exchange Areas/Private Messages etc.? [Consolidated]
So, I guess I meet all of the qualifications to send private messages, except the last one in the link you provided. So, I'm not sure what I need to do in order to be able to send PM's? I've already replied to PM's without any trouble that were sent to me by other members, but in order to send my own PM to another member, I need to meet this last piece of criteria since I already meet the other two?