I've just added a Macintosh-specific version of Junicode 0.6.15. It is functionally the same as Junicode for Windows/Linux, but perhaps a little easier to install, and tested in Mellel and XeTeX on the gorgeous MacBook that my department just bought for me. You must have OS X 10.4 ("Tiger") or later if you want to see the pretty OpenType things.
http://sourceforge.net/project/showfiles.php?group_id=158636
Peter