A “PM” is a Personal Message, NOT A PRIVATE MESSAGE. The PM feature is available only if you have reached the proper Trust Level. To send a PM, click on your avatar, then click on the envelope icon, then click on the New Message button. Input who you’re sending it to, what the subject is, then write your message and click the Message button at the bottom.
Note: PM’s on this site will have a light green palette around them, distinguishing a PM from a regular post which is displayed publicly to all users.