I always wondered why Hagrid never told Harry how to get onto the platform. Well perhaps it was because he knew that Petunia Dursley knew how, and expected her to explain to Harry how to walk through the barrier. In HP and the Deathly Hallows, inside Snape's memory, Petunia was with Lily and their parents on platform 9 3/4. So she obviously knew how to get there. So, why not explain or tell Harry how? In HP and the Sorceror's Stone, Uncle Vernon asks where the school is. Harry checks his ticket and it doesn't say what part of the country, it just says to take the train at 11 o'clock on platform 9 3/4. Petunia was in the room while he tells them this. So she never explained to Harry. Why? Because she's always denying she knows anything about the magical world? Or was she just being crude?
I can think of a few possibilities:
The simplest explanation of all. The time(s) she went there with Lily were decades ago, and part of a section of her life which she'd done her best to forget and excise from her memory. The woman who Harry grows up with isn't the same person as the girl who wanted to be able to do magic like her sister. Perhaps she's blotted out all of that part of her life.
She didn't want to reveal any knowledge of the wizarding world.
Especially in front of her suffocatingly Muggle husband. I doubt she ever told him that she'd once longed to be able to get into Hogwarts. He seemed as shocked as Harry when she got the Howler in book 5. Surely she wouldn't want to mention in front of him, at this early stage, that she'd actually had more involvement with the magical world than she'd admitted. Even if she did remember, she might not want to reveal that fact.
She's being petty and spiteful.
Why would she want to help Harry out? Let him get stuck at King's Cross not knowing how to access the platform. Then maybe he won't be able to study at Hogwarts and will have to live a Muggle life like she'd always wanted him to. She's certainly not going to make it any easier for him to go off into the insane world that claimed her sister's life.
Petunia hates wizards and witches! She hates Hogwarts, and had, up until that time, tried to deny any knowledge of their existence.
They had already taken great pains to prevent Harry going to Hogwarts. They were terrified by Hagrid's visit and absolutely would not want to help Hagrid in any way.
Fear leads to anger, anger leads to hate, and that way, the dark side lies.
The Dursleys hate magic, they also hate Harry. She doesn't want Harry to become a wizard, or to help him prosper in any way.
However, they could easily abandon him at Kings Cross station, saying he has a train to catch, which he did, but keep him away from the wizarding world by keeping how to get to the platform from him.
It's all pretty clear from the outset.
I'd add a fourth possibility to Rand al'Thor's answer.
- She never knew
She was on Platform 9 3/4, but that doesn't mean she knew how to get there, in the same way that being driven somewhere doesn't mean you have the knowledge to drive a car. She may have assumed other magic was at work beyond "run at it".
An additional possibility:
She did not think it would be a problem
After all, she had been there, and presumably she got on without a problem, because somebody was waiting for them or because they were guided. She had no reason to think it would be different for Harry.
This combines with all the other reasons (unwillingness to reveal she knows anything in front of Vernon, unwillingness to help, simple unwillingness to intervene, or simple ignorance or forgetfulness of the method to be used together with a reluctance of saying something incorrect).
In fact, I would say that she has no reason at all to tell Harry.