Here is a list of the current unofficial level sets for Neverball and Neverputt. Visit their corresponding forum thread to report any feedback.
Any ZIP archive that has been packaged properly can simply be placed into your user data directory. Neverball and Neverputt will read the sets and courses in each of these archives.
Note: If you use Nuncabola, put the archives into the “Data” subdirectory.
Under Unix, Linux, and OSX this directory is called ”.neverball” and is created in the user's home directory, specifically, the directory set in the HOME environment variable.
Under Windows it is called “Neverball” and is created in the user's application data directory, the location of which is obtained by inspecting the APPDATA environment variable. An easy way to access this directory is by typing %APPDATA% in the Windows Explorer address bar. If the location could not be determined, it is assumed that the user has permission to write to the game directory, and the user data directory is created within.
Not all sets or courses may be up-to-date with the 1.5.2 format (1.5.3 for Neverputt). Such sets for Neverball might not work correctly in 1.5.2, and such courses will not work in Neverputt 1.5.3 or later (you will lose some holes).
The reverse also holds true, if you possess an older version. Sets made in the 1.5.2 format will still need to be manually extracted, and added to sets.txt. Courses made in the 1.5.3 format will crash if loaded in 1.5.2 and earlier.