How does Nora in Ibsen's play A Doll's House epitomize the id in Freudian theory?

Based upon Freud's definition, the "id" is that which is most concerned with the present moment: what feels good and what makes us happy at the moment.  It is not based upon a clear perception of the world around us, but is much more child-like.

With this in mind, Nora is a character that does whatever it takes to survive in Torvald's world at the moment.  She is like a doll.  She is playing house. She dresses up to please him.  She lets him treat her like a child that has no idea about the "real" world, and in fact, she does not.  How could she, Torvald would wonder, as he sees her as a mere woman with no capacity to understand the complexities of a "man's" world.

Even when Nora borrows the money to save Torvald's life, she does not realistically ask herself what additional "cost" there may be in dealing with the unscrupulous Krogstadt.  Later, as Krogstadt points out to her the crime she has committed under the law, she admits that she has no concern for that reality: all she cared about was saving Torvald's life, and if the law finds fault with how she did it, the law must be wrong.

The child-like impulsive nature of Nora's behavior is seen, too, with how she acts with Dr. Rank.  She flirts with him, but thinks nothing of it because although she is a grown woman, she is locked in that unrealistic "id" place, where she simply, naively plays the little girl, seeing nothing wrong in showing him, for instance, the stockings she will wear to the party.  This would have been terribly inappropriate for a woman at that time in history, but she teases it away, with no realization that Dr. Rank is a grown man and she a woman, married at that, and that this might be seen as a seductive move on her part.  These kinds of behaviors are what Dr. Rank admits so confuse him about her true intent towards him.  In truth, she has no intent at all: she is totally unaware that she is acting inappropriately.

Mrs. Linde is the voice of reason in the story, showing Nora the hard realities of life, using her own terrible experiences to show Nora how out of touch with the world she is.  She tries to explain to Nora that she is very lucky not to have to deal with the harsh world confronting a woman on her own.  Nora brushes this aside, with no concept of what Mrs. Linde is talking about.

However, when Nora finally realizes the grave nature of her relationship with her husband, that he cares more for his reputation and nothing for her sacrifice to save his life, reality comes crashing in and she awakens, as if from a dream.  Here the ego asserts itself.  However, she is not completely "healed."  As Nora decides to leave Torvald, even with the knowledge Mrs. Linde has shared as to how difficult it is to live alone as a woman in that era, the "id" is still alive and well.  Nora goes without a realistic idea of how she will survive on her own.  What she does know is that she can no longer live like a "doll" under Torvald's roof.