This is briefly described here:
http://leecher.freddy.lt/faq.php?expand=faq405The process is like this:
0. You can set in "Options" -> "Messages" -> "What to do when sending messages to themaPoster" -> either post immaterially (default) or add to posts list.
1. After you click "Send to themaPoster" you will see what can be seen your screenshot. Choose "reply" (to topics) or "create" (new threads). Press "OK".
2. After that you will see group or sections (they must be setup in themaPoster, if you're using it you probably already have some topics or templates there). Choose needed.
3.
Only after that the post will be sent to themaPoster and you will see it in themaPoster.
Based on settings (at step 0) you should see the post in"status" -> "active" or in "posts list".
Both programs must be running on the same computer.
Also from your screenshot the message is completely empty. That might be the problem. It should not be empty.