To answer the question, one needs to first understand the elements of a sentence.
A subject of a sentence is what the sentence refers to. The predicate of a sentence tells readers something about the subject. In regards to the sentence in question, the predicate is notated by bold face:
That is Rachel with the ball.
A nominative is depicted as another name for the subject of the sentence.
Therefore, in the sentence in question, the nominative will be the word which refers back to the subject of the sentence.
That is Rachel with the ball. That refers to whom, or what the sentence is about. The sentence is stating who Rachel is--she is that person. Therefore, the predicate nominative of the sentence is Rachel given that that refers to Rachel and not the ball. The ball is an identifier; it is helping the reader, or to whom the sentence is being spoken to, to recognize which person is Rachel.