I figured it out. It was operator error...though in my defense, it seemed somewhat unintuitive.
I live in Illinois. So when I tell it I want to enter a destination, it defaults to "United States, Illinois". It then asks for a city or zip code. I thought I should be able to just type in the zip code for anywhere in the US, but it doesn't work that way.
No, I had to first change my current location to "United States, Wisconsin" before it would allow me to enter an address in Wisconsin. That was the only way to do it. None of the other GPS systems or apps I've used work that way; this is what was throwing me off. So at least I know how to do it.
For my upcoming trip to Ohio, I had the addresses of both people I'm planning to visit in my phone's contact list. It was easy to import each of them as a Favorite in the car's nav system, so I can get to them with just a couple of button presses.