World files can be loaded into Minecraft using a plugin called Multiverse-Core (for servers). To load a world file in singleplayer, simply unzip the initial ZIP/RAR file and place the resulting folder inside the Minecraft saves folder.
Video tutorial (Multiverse-Core): https://youtu.be/nAHZeLnH3Xk
Multiverse-Core plugin: https://dev.bukkit.org/projects/multiverse-core
To load a world file in Minecraft on a server or singleplayer:
Upload the .ZIP file to your server's file management system.
Unzip the .ZIP file into your server's root directory.
Verify that a new folder has been created.
In either the server console or in-game, execute the following command:
1. "mv import foldername normal" - Do not include a leading slash (/) in the console command.
2. If the command is successful, you will see a message in the console or in-game chat.
3. To set the new world as the default world that players spawn in, deactivate the server.
4. Delete the existing world folder and rename the new folder to world.
5. Activate the server.
To load a world file in Minecraft in singleplayer:
1. Unzip the .ZIP file to your Minecraft saves folder.
2. Start Minecraft and select the new world from the list of available worlds.