How do you specify which zip version to compress with?
Years later, but in case someone else comes across this, Canvas has written a very useful guide to .imscc files. It discusses why they're problematic on OS X:


  It won't work on a Mac, because the Mac adds junk hidden files to the zip file that cause the cartridge to not be recognized.  There might be a work-around for getting zip files without extra junk on the Mac, but I don't know it, so we just used Windows instead.