I believe the answer is: She made friends with the creature by throwing him a red rubber ball.
Despite its scary features, Cerberus is basically a pet dog that is told to guard the gate. (even though the gate that it protects is the gate of hell rather than the gate of normal household).
So annabeth instinctively think that Cerberus must be bored since He got no friend and attention in that place and play fetch with the ball that she stole from the Waterworld gift store.
