Hey! Contrary to the "best answer", the sentence is fine. It's just more of a poetic grammar structure, found in poems, quotes, or proverbs. You could also read it like this:
"He (who is) not busy being born is busy dying."
I personally interpret the sentence as a way of saying that if someone isn't discovering new things, letting their creativity flow, or living life, they are dying.
As Randy said though, there is no one way to interpret art :)