On Mac:
1. Find your Minecraft folder
To find your Minecraft folder:
- right click FInder and then click "Go to Folder"
- type ~/library/Application Support
This video shows how to get to your Minecraft folder:
https://www.youtube.com/watch?v=Wi2fhQfmTjc&t=16s2. Scroll down and find "minecraft" (no caps)
3. Drag that folder to the sidebar.
4. Find your downloaded .zip file.
5. Drag the .zip file into the Minecraft folder in your sidebar.
6. Drag the zip file into the folder titled "saves" in your Minecraft folder
Relaunch Minecraft and go on singleplayer!