What did Jean-Paul Sartre mean when he said that because there is no creator, humans are "Condemned to be Free"?
Human being is freedom.
The external world is filled with in-itself being. Consciousness is the only anomaly, and consciousness only manifests itself through human being, insofar as we are aware of it. So, the starting point for an account of human being is in the account of the being of consciousness. We know that the being of consciousness is the consciousness of being, and it will follow that the being of consciousness of humans is the consciousness of being human. This consciousness of being human entails a consciousness of consciousness, and the immediately apparent features of consciousness. Conscious is infinite which means that the options for manifestations of human consciousness are infinite. The feature of having an infinite amount of options for the manifestation of one’s consciousness is what Sartre calls freedom. So, we have determined that the human being is freedom and that humans must be aware that their being is freedom.
This does not mean that human beings have an essence of freedom. “Man does not exist first in order to be free subsequently; there is no difference between the being of man and his being-free” (BN: 60). Freedom is the reason that human beings do not have an essence. “Human freedom precedes essence in man and makes it possible; the essence of the human being is suspended in freedom” (BN: 60). The freedom of human being manifests itself as the limitless choice of human action. No matter what the situation is, a human being can always choose to act and his action will define his being. Even in extreme situations of coercion (such as being threatened with death), a human being still has the ability to choose his action and to choose the conscious attitude with which he apprehends the world. This ability to choose is actually an inability not to choose. Sartre tells us that “we are a freedom which chooses, but we do not choose to be free” (BN: 623). We are not free beings. We are freedom itself.
Citations refer to this translation of Being and Nothingness
I don't want to simplify, but this almost seems to be a logical blatancy: As a human being without a moral superior, you are forced ("condemned") to choose. Even not choosing is a choice. A moral superior provides a) rules you have to follow to achieve b) a certain objective. Following these rules and knowing what you're doing it for often makes choice a lot easier. Which is why the atheist is endued with condemnation - because it makes his/her life so much more troublesome and complicated.
(Disclaimer: beginner's view here. Proceed with caution.)
In the views of Sartre, the world's nature is strict and rigorous, even on a metaphysic point of view. Even with a dualist if point of view and certain influence ("each thing is defined by what is not that thing") he realizes that the true nature of every existence is bound by the rules of that nature, and there is no way in which that may change. If it changes, then it was not true essence.
As such, human seems to be different, because our awareness allows us to have free will and choose what our actions will be. If we can accept at this capability comes from a superior being that transcends our nature, then there's a chance, by those divine ways, that we may transcend it as well. In the absence of such a chance, human nature is, in Sartre's point of view, not different than any other nature, and our free will is nothing but an illusion of choice. (This concept would be enforced by behavioral psychology.)
In a way, we are forced to exist, and we are forced to be who we are. We may choose to change, but as so, we were meant to change, as it was in our nature all along. We are then, "condemned to be free" and forced to choose, because it is in our nature.