It is not officially supported on 64-bit. The install-readme.html file that comes with the current release (6.0 SP1) specifically states this (slightly different from what you have in your install-readme):
Microsoft Windows XP, Vista, 7* or 2008 Server
* Windows 7 support is limited to 32-bit operating systems. Although no problems have appeared with XMetaL Author Enterprise under Windows 7 64-bit, please use XMetaL Author Enterprise at your own risk or check with Customer Support if you have any concerns.
Despite what that says (which is referring to our internal testing) and including the fact that we have reports from clients saying they have no issues running on 64-bit either, we also have reports from clients stating they do have issues. Our QA team will be testing Windows 7 64-bit for our next major release (probably numbered 7.0) and we plan to officially support it.
Microsoft states that Windows 7 64-bit is supposed to run 32-bit applications without any problems -- in theory -- but if you do run into any trouble I would recommend using "XP Mode" for Windows 7. More information about this is located here:
http://www.microsoft.com/windows/virtual-pc/download.aspx