In "The Necklace," I need three quotes from the story to show that Madame Loisel is ungrateful.
There is certainly plenty of evidence to show Madame Loisel's dissatisfaction with her lot in life! You might want to start out by examining the beginning of this story and the way that she is presented. Consider, for example, the way that she is shown to "grieve incessantly":
She grieved over the shabbiness of her appartment, the dinginess of the walls, the worn-out appearance of the chairs, the ugliness of the draperies. All these things, which another woman of her class would not even have noticed, gnawed at her and made her furious.
It is clear from the way that the narrator tells us that "another woman of her class" would not even have noticed such things that Madame Loisel is profoundly dissatisfied with her position in life.
In addition, you could use any of the lavish daydreams that fill her day at the beginning of the story to indicate that she is ungrateful. She clearly does not value what she has, otherwise she would not invest so many hours in fabricating fantasies. Consider, too, her response after visiting her friend who has done much better in life:
She had a well-to-do friend, a classmate of convent-school days whom she would no longer go to see, simply because she would feel so distressed on returnign home. And she would weep for days on end from vexation, regret, despair and anguish.
Such an emotive response is rather hyperbolic, again reinforcing the profound ungratefulness of Madame Loisel's character. She is very comfortable in life, but her desire for greater wealth and prestige means that she is blind to how lucky she is.
"Instead of being delighted, as her husband hoped, she threw the invitation on the table with annoyance, murmuring". - She did not even acknowledge how hard her husband worked to get her the invitation.
"The day of the party drew near, and Mme. Loisel seemed sad, restless, anxious. Yet her dress was ready." “It annoys me not to have a jewel, not a single stone, to put on. I shall look like distress. I would almost rather not go to this party.” - Even after her husband sacrificed the money he had to buy his weapon to buy her a dress, she still found a reason to complain saying she did not have jewelry.
"She was simple since she could not be adorned; but she was unhappy as though kept out of her own class;" As beautiful as she was, she was still not pleased.