As has been stated far too many times than should be necessary:
Quote from: TerryEnjoy! To distribute your level for other people to play, go find the .VVVVVV file in your Documents folder. (On windows, that should be in My Documents/VVVVVV, on Mac it's Documents/VVVVVV, on Linux ~/.VVVVVV). You can play other people's levels by copying files into this directory.