Are saves portable?
The world saves are done in the same format, so to transfer a world you just need to copy the folder with the name of your world across.
This can usually be found here on macs: (username)/Library/Application Support/minecraft/saves
Copy the entire folder to your windows files. Windows file structure depends on version but commonly it's found at: C:/users/(username)/appdata/roaming/.minecraft/saves
appdata May be an invisible folder by default, you can make it visible by changing your settings in the windows file viewer.