Most of the time you can you use 'at' or 'in'. 'At' just means where you're located ie. at the shops, at the kitchen etc. 'In' means what you're inside and most of the time is more common. "She is at THE kitchen and is making breakfast." This is the correct sentence. Even better: "She is at the kitchen making breakfast."

The correct sentence should be: She is in the kitchen (and she is) making breakfast. Sometimes you can use "in" or "at". I would say... "I'm in the kitchen" [inside the room] "I'm at the kitchen door" [next to the door] "I'm in the cupboard" [inside the cupboard] "I'm at the cupboard" [next to the cupboard] If you said "I'm in the shop", you must be inside, but "I'm at the shop" could be inside or out. "I'm in the car" would be inside it, while "I'm at the car" would be outside it. Just remember "in" always means "inside" and "at" could mean "inside of" or "outside of".