If they contain an installer package (like the Snow visualizer described above), just run it and you’re all set. (Admin password required.)

If the downloaded file has a .qtz filename extension (as does the punkt package described above), place it in /System/Library/Compositions (Admin password required).

If the file has a .bundle filename extension, put it in the /Library/iTunes/iTunes Plug-ins folder (Admin password required) to make it available to all users of your Mac, or in ~/Library/iTunes/iTunes Plug-ins to make it accessible only in your user account.

Some visualizers may contain both .qtz and .bundle files; if they do, both components must be installed as described above.

You’ll have to restart iTunes (and possibly your Mac) before the new visualizers are listed in the View -> Visualizer submenu in iTunes.

Remember to press ? when your new visualizer is running to see its option controls.