When she was told to go to him, Iris wasted no time.
It had actually been a long time since Zeus had called her into his presence. She was looking forward to seeing him again, to receiving his orders and carrying them out. Getting to him wouldn’t have been the easiest thing, but Iris was fortunate enough to be able to make flight arrangements as she pleased. If Zephyrus agreed to bring her to Atlanta for any reason other than her asking, Iris didn’t give it too much thought. She was used to his protectiveness, though with Zeus, she knew her husband didn’t have to worry about her.
As for knowing he was in Atlanta, that wasn’t difficult. It was Zeus, and he wanted Iris to find him. Once there, she found her way to him unerringly, not pausing until she was at his door. If she hadn’t been completely confident that he wanted her there, or that she had every right to be, she might have hesitated, but she didn’t. She knocked and waited for his permission to enter.