Edit: MC version 19.0.131 can directly import Mac iTunes's XML playlists - there's no longer a need to use this script for those playlists types, unless you also want to change path components as well (using -m transformations).
Here is a script to convert M3U's or MPLs bidirectionally between Mac and Windows. It will also convert iTunes XML playlists so that MC can import them (currently Mac only, until I get a Windows version of an XML file to test).
The perl script is attached. Download and unzip it. To run it, it works like this:
perl convMCplaylist.pl -m 'old|new' path ...
You specify old and new; where old is the part of the file name in the playlist that you want replaced with the new. Typically these will be common base paths, such as:
/Users/mrc/Desktop => M:\Testing\Music
Consider the file paths:
Mac: /Users/mrc/music/artist/album/track.mp3
Win: M:\Testing\Music\artist\album\track.mp3
The -m to use would be:
-m '/Users/mrc/music\|M:\Testing\Music\'
You can use as many -m mapping transformations as you want; they are applied in the order you specify them. They apply to all lines in the file, so be careful.
If you don't need any additional mappings beyond those done automatically for specific playlists types (i.e. for iTunes' XML conversion), use:
-m -
The path argument is one or more playlists and/or folders containing playlists (m3u, m3u8, mpl).
The script handles swapping path separator characters for you.
The playlist files will be changed in place, and the originals will be saved as <playlistname>.orig. If you don't want the backups, remove the ".orig" at the top of the script (leaving "#!/usr/bin/perl -i"). Still, you might want to copy your playlist folder just in case something goes wrong for easy recovery.
Once converted, you should be able to bring them over to the other platform and use File > Import Playlist to import them.
Examples:
perl convMCplaylist.pl -m '/Users/mrc/Desktop/|M:\Testing\' ~/Desktop/test
perl convMCplaylist.pl -m - ~/Desktop/iTunesPlaylist.xml