Fusterlandia is in the town of Jaimanitas which is east of Havana. You can get there by taxi.
It would probably cost you about 20cuc to drive there in a yellow taxi. Just stop a taxi in Havana and tell them if they know where Fusterlandia is in Jaimanitas. It’s well know and Jaimanitas is a tiny town anyway, some every driver should be able to take you there.
You could also get there in a standard communal taxi, but you would have to know the route… Basically you would take a taxi going east on Linea street and say you want to go to El Paradero… which is the slang name for the bus stop in Miramar (near Cony Island, -Isla de Coco). And from there, you would take another taxi going to Jaimanitas. It would take longer to get there by communal taxi.. But it would only cost about 2$ per person.
As for getting to the Hemingway house, which is called La Finca Vigia, it is located south of the city, about 10 kms. You would have to take the highway. While it is possible to do it by communal taxi, it is rare. You would have to look for a taxi in Old Havana that is heading to the Carretera Central (The highway). You would have to ask the driver to drop you off at the Finca Vigia. If you were Cuban, you could probably get there for about 2$ or less, per person. But the second the communal taxi driver sees that you are a tourist, the price will likely jump to 20$. And at that rate, you would be better off taking a standard yellow taxi to get there.