You are getting the error message about your post count being too low because it is. I think you are confused about the difference between a PM and a post.
A PM is just that; a private message between you and another DISer. A post is when you begin a thread or reply to a post on a thread. PMs do not count towards your post count.
You cannot PM another DISer until you have a certain number of posts. This is why you are getting the message that your post count is too low to send a PM. Its been a while since my post count was low, and I can't remember the exact number of posts you need to be able to PM someone, but I think you are almost there. Your post count as I type this is 9. I think you have to have somewhere between 10 and 20 before you can send a PM.
Hope this helps.