How did she know that there was a comb machine in the mens room?

It was no mystery, really. It was common back then (and perhaps still is) for those sorts of roadside diners to have a vending machine in the men's room that would sell various toiletries that a traveling salesman or trucker would need, while on the road. Kitty knew this, and simply sent Dick off to the men's room to buy a comb so as to get him out of the way while she robbed the diner.

I guess the only "mystery" is the way in which Dick seemed confused that Kitty knew there was a vending machine in the men's room. He seemed unable to grasp that Kitty (or anybody else who'd had any experience on the road) simply knew these sorts of things. You're right about Dick being a dim bulb. He didn't seem to have much practical experience with people.

One scene that made me laugh was when Dick, Kitty and Mary Fenaka were in the camper, and they were listening to somebody obviously walking on the roof of the camper. Then Dick turns to the other two and says (like it's some big revelation): "There's somebody on the roof!" There were lots of times in the film like this, when Dick didn't really seem connected to reality, or other people.

Yeah what Eric says. I think they never explained that further because there isn't much explaining to do. Those things were probably everywhere at the time, like condom machines in men's toilets today.