MSST does not work on Macs. I have found no equivalent suitable mapping software that does. For sharing, my solution is to send excel stitched maps or pdfs of a finished map/route.
For last two years, I've operated a Mac at home, so I switched the MSST software to work laptop. Recently installed Bootcamp at home, so I have both Windows XP and Mac OS running. It's the best of both worlds, as I can do all the mapping I want on the Windows side with Microsoft Streets & Trips or DeLorme Topo, created PDFs of the finished products and then share from the Mac side.