That is correct. You CANNOT use PM system to send messages to other users unless you have 15 or more posts. After deliberation, we agreed to put the limit on 15 posts, however I would have put the limit to 50 posts as it is on DefenceTalk.com!
Why?
The answer is simple: DefenceTalk.com and other forums (such as PDF, WAB, Keypublishing, china-defense, Pakdef, etc.) suffered from PM spam attacks by those who are busy cloning DefenceTalk as well as prominent pakdef sites (they cloned Pakdef and now busy cloning DefenceTalk as well as SDF and SinoDefence.com, the clone of latter was shutdown after I contacted their host), and we had to implement this restriction in order to control useless spam and extra useless utilization of the website resources.
I wish to take the restriction up to 50 posts and that may happen someday but for now it will stay at 15 posts or more to use the PM system.
As far as asking questions about the forum: All questions related to Forum, its function, its backend, its frontend, how it works and any complaints should be directed towards me. Unless you have question which strictly relates to one of the moderators then it should to go that moderator(s). If you have question related to chinese Defence issues and SinoDefence.com, those questions should be directed to DongFeng, who is one of the administrators here as well as on his site, sinodefence.com.