You only have 3 posts, according to the count on the left. And you have to be registered on DISboards for at least 24 hours for the PM option (called "conversations" here) to be available.
You can reply to existing threads and you post count will increase. But please, don't bump decades-old threads. Responding to the conversation on any current thread works just fine.
A "post" is generally on a public thread, just like replying here.
Depending on context, a "message" might be what is also known as a private message, PM, conversation, etc. It's like an email or IM, just a message between you and one (or a few) others, not public. On this forum it's officially called a "conversation" and it's the little envelope next to your username in the upper-right of the dark blue menu bar, next to the bell for Alerts.