The following is from the FAQ section:

"Airline Pilot Central recently changed the forum policy for the Private Message (PM) feature. Registered users are now required to have 25 public posts before the site Private Message feature is available. The system looks at the number of posts (25), and the aggregate word count of all posts. If a user makes 25 very short posts, the system may raise the threshold for that user to a higher number than 25 posts. Best bet is just keep making the quality posts - just more of them! To fill the gap left by the PM threshold, I've added the ability to use Instant Messaging. The first thing you should know is that IM providers are NOT affiliated with this site. Provide you IM "handle" at your own risk. All the popular IM clients are represented as well as Skype! Add your IM details in the User Control Panel>Edit Profile. Enjoy!"