How do I change a multiplayer world into a singleplayer world? [duplicate]

There is a simple way to combine your Multiplayer world for Single-player use.

Notice how your single-player worlds are saved in region, DIM1 and DIM-1 folders. Each of these folders is your dimension.
Multiplayer servers saves each dimension in its own folder, while the game (single-player) saves it as a whole folder of its own.

So, what you want to do is copy the DIM1 (Nether) and DIM-1 (The End) into the folder with the overworld dimension in it (level folder; as it is called in your screenshot), and then you can put your level world folder straight into your saves folder, in the Users/[Username]/Library/Application Support/minecraft/ folder, which then will run perfectly fine with all your dimensions intact.

Player inventories, locations etc. are stored in the overworld folder (level), so all locations will be preserved. (Means if you were in the nether, you'll log back on into the nether, or where the level.dat says you are)

You should end up with a file directory like such:

...
    - local
         - ...
    - minefold
         - ...
         - level
             - data
                  - ...
             - DIM1
                  - ...
             - DIM-1
                  - ...
             - players
                  - ... 
             - region
                  - ... 
             - uid.dat
             - level.dat
             - level.dat_old
             - session.lock

Note: "..." means things not included/not important.

The DIM1 folder contains data for your Nether,
The DIM-1 folder contains data for The End and
The region folder contains data for your overworld.

Everything else, you do not need to touch.