by 'structure', they meant that you used no punctuation or made any attempt to differentiate who was speaking; rather, you've put both sides of the characters' dialogue on each line. Better example:

"Well, what did you have for breakfast last Tuesday, then?" asks the old man. His co-worker replies, "Porridge, I always eat porridge for breakfast."

"Well, I always heave my breakfast!" said the old man.

Also, once a dialogue is established you can then alternate without stating who is speaking.

"You heave your breakfast?" asks the co-worker.

"Yeah. I'm an old man."

"With a bad memory?"

"Shut the fuck up, Greg!"