This topic was discussed in this forum thread over on our forum site, and 'mark hunte' came up with a method to view these files using only your Mac, Stuffit Expander, and TextEdit. Then user Hayne stepped in, and trimmed a few of the steps by using Graphic Converter. Although the process isn't exactly a one-click operation, it's not too bad, especially if you have Graphic Converter.
So if you have an MTHML file you need to view, check out the above references to make it usable on your Mac.

