CannotResolveSymbol said:
Then they shouldn't be forcing the 8 people running XP x64 to upgrade...
What makes you think the other 7 are running WLM? 
In all seriousness, supporting different OS's is a potentially expensive process, so it's hardly surprising that niche products like XP x64 aren't in the support matrix of what is generally intended as a consumer application, in the same way that it isn't likely to be supported on a server OS.
Notably it installs and runs fine on the more consumer-oriented 64-bit operating systems, Vista and Windows 7.